Ooh, Hobbit recut? Sounds exciting! Are you excising the White Orc entirely? Seems like that'd be tricky to pull off.
We actually watched most of B5A recently (it was on TV), and I realised then that you probably could cut together a 95% book-faithful Hobbit movie from the trilogy; they mostly added stuff, rather than totally rewriting it. It'd be tricky to make Tauriel and Legolas go away completely, though.
-- what's that? Minas where? Which Hobbit film is that in? XD
Looking through RotK again, I ran into this passage:
He went out, and soon after all the others followed. The day was still fine, though it was growing hazy, and it was hot for March, even so far southwards. Pippin felt sleepy, but the lodging seemed cheerless, and he decided to go down and explore the City. He took a few morsels that he had saved to Shadowfax, and they were graciously accepted, though the horse seemed to have no lack. Then he walked on down many winding ways.
People stared much as he passed. To his face men were gravely courteous, saluting him after the manner of Gondor with bowed head and hands upon the breast; but behind him he heard many calls, as those out of doors cried to others within to come and see the Prince of the Halflings, the companion of Mithrandir. Many used some other tongue than the Common Speech, but it was not long before he learned at least what was meant by Ernil i Pheriannath and knew that his title had gone down before him into the City.
He came at last by arched streets and many fair alleys and pavements to the lowest and widest circle, and there he was directed to the Lampwrights’ Street, a broad way running towards the Great Gate. In it he found the Old Guesthouse, a large building of grey weathered stone with two wings running back from the street, and between them a narrow greensward, behind which was the many-windowed house, fronted along its whole width by a pillared porch and a flight of steps down on to the grass. ~Return of the King
At first I was terrified that this meant that the idea of a main road through the city was wrong, but the fact that Pip is 'explor[ing] the City' means he can just not take the direct route. What it does mean is that the side-streets are much more substantial than just 'alley between two buildings', as the movies would suggest: the Lampwrights' Street runs towards the Gate, meaning it must run parallel to the wall.
Falling back on maths, from the 3100 foot diameter we can estimate that each circle is some 220 feet across. Somewhat arbitrarily, we'll call the walls 10 feet thick (Chester's are ~6ft), and guess the main road at 30 feet (Chester's high street is a wide 35, but space is a premium in Minas Tirith). That leaves 180 feet for buildings, or 90 feet on either side of the main road.
30 feet is also a reasonable figure for the size of a medieval building: a one-room house would be about half that, so we're looking at two rooms deep. In that 90 feet, then, we can comfortably fit two rows of houses, a road, and maybe an alley or two. Something like this:

Tolkien tells us the First Circle is the widest, so the upper ones are probably just one building either side of the road (or maybe the road runs against a wall), while the lowest circle would have space for another wide rank of buildings at the base of my image. That would explain why the Old Guesthouse, which seems pretty chunky, somehow doesn't face onto the main street.
Right! Having spent far too many words on that, let's consider those markets. Here's an example from Chester:

Tolkien talks about 'arched streets', so we can imagine that the raised market/walkway crosses over the various side roads on bridges. Note that these are the main steps up; there are other, narrower and steeper flights spaced along the walkway. You can picture any number of little shops spaced along the walkway, selling whatever is needed day-to-day. If we allow Gondor to work a little on the Roman model, the people might have the habit of eating out almost every meal, with food at home being unusual. I believe Rome had communal kitchens for if you did need to make something yourself, which would make a lot of sense in a city as pressed for space as Minas Tirith.
This stuff would be spread throughout the city - you don't want to walk a mile in the morning just to get your bread! - and in the more cramped circles they might do away with the walkways entirely, operating solely from the street level. Presumably the shops would mostly front onto the main highway, while the houses, where possible, would be set back.
So much for the daily shopping; what about the weekly market? Food would be brought in from the farmlands around, and there'd be a flurry of purchasing. Where would you put that?
Leaving Chester behind, let's look at Bath (never much of a hardship for me). They've taken an unusual approach to fitting the market in - it's actually underground, or at least under another building (the Guildhall). So picture your civic structures, your guardhouses and judicial courts, and yes, your guilds or their equivalent, built up to the first level, with large open areas underneath them that are only opened up on Market Day. They might be fully enclosed, or the buildings might just be held up on pillars, leaving the space as an empty pavement the rest of the time.
Where would this be? My guess is fairly low down. The bulk of the population will be in the lowest circles, and you want to centre your market on the people. The high-and-mighty lot up in their literal ivory tower can drag a handcart down and pick up their vittals just like everyone else. ^_^
There's one more market we need to consider, and it rises out of the fact that Minas Tirith is not the economic centre of Gondor. That's Pelargir, which was a massive trading hub. It almost certainly shipped goods both up and down Anduin, likely in bulk (at the least you want to fill your ship), and when those goods arrived in Gondor, it would have been huge.
People being generally lazy, the monthly(ish) Market would have its centre at the Great Gate. Tolkien tells us of a 'great paved space into which all the ways to Minas Tirith ran', and that's got to be a market square. But, people being prone to doing anything they can to get a sale, it would have spread into the city proper, as a line of stalls up the centre of the road. With a 30ft highway, you could slap in a 10ft tent and still leave room for people to squeeze past.
I don't imagine they'd spread past the first circle - that's too much effort - but up to that point, the city would be absolutely packed. The Old Guesthouse was probably one of many, mostly quiet except during market time. You'd have traders coming in from Rohan (it's easier to reach Minas Tirith than Pelargir), and even - in happier times - from the closer parts of Rhun. They'd straggle in over a week or so, join in with the trading rush on Market Day, then wander away when all their business was concluded.
... you only asked one question about markets, didn't you? Um... oops. Mooooving on:
Guests would probably stay in the same general area as who they were visiting. A guest of the court would be lodged on the Seventh Circle - Gandalf and Pippin were. If you were visiting a merchant on the Fourth, to invent an example, she'd probably recommend a nearby inn or lodging-house, or offer her guest-chamber. You don't want your guests to have to trek up- or downhill to see you, after all!
Spiral staircases can be wide or narrow, but yeah, doesn't sound like fun with a cane. Luckily, the only attested stair of that sort is in the White Tower itself, so unless you're important enough that the Steward asks you to take a room in the palace, you're probably safe.
As far as mobility aids go... I have no idea. :) Delta's said more than I could have, so I commend you to her. My Harlequin Took doesn't have any specified means of transportation except for 'friends'.
--no, hang on, I take it back! Wheelchairs are canon to Middle-earth, and are key to the story of Pippin's sister the political assassin:
Lalia [Took], in her last and fattest years, had the custom of being wheeled to the Great Door, to take the air on a fine morning. In the spring of SY 1402 her clumsy attendant let the heavy chair run over the threshold and tipped Lalia down the flight of steps into the garden. So ended a reign and life that might well have rivalled that of the Great Took.
It was widely rumoured that the attendant was Pearl (Pippin's sister), though the Tooks tried to keep the matter within the family. At the celebration of Ferumbras' accession the displeasure and regret of the family was formally expressed by the exclusion of Pearl from the ceremony and feast; but it did not escape notice that later (after a decent interval) she appeared in a splendid necklace of her name-jewels that had long lain in the hoard of the Thains. ~Letter 214, J.R.R. Tolkien
So there you go: characters unable to walk can have wheelchairs, provided they do not mind the possibility of being thrown out of them to their deaths.
hS