First up, this version of Maglor is significantly less sympathetic than Dafydd. He starts out trying to kill Susan outright, and only relents because he's seriously injured and he has the weapons. Oh, he's moderately charming, but so's a tiger. Doesn't mean he's not going to bite you. Ultimately, yes, he leaves her alive - but after what's probably the longest conversation he's had with anyone (other than Tolkien and Shakespeare) for hundreds of years.
But leaving him aside, and despite the fact that your script version is hilarious, I'm not convinced by your perspective on the eldest Feanorians. We know most about Maedhros, so let's look at him:
-Maedhros was friends with several of his relatives - notably Fingon. This friendship persisted after the first Kinslaying, so we can't peg it on 'hiding his true colours'. There was clearly something worth liking.
-Maedhros was the only son of Feanor who stood aside from the burning of the ships at Losgar. Given that his maniac of a father was right there, that took a certain level of moral courage.
-Maedhros actually yielded the High Kingship to Fingolfin; yes, this was for political reasons, but I can't see most of his family doing the same (and I dread to imagine how Caranthir took it).
-Unlike certain of his brothers, Maedhros was committed to taking out Morgoth. He stayed on the defensive line after the Bragollach - unlike Cel'n'Cur, who ran as far as they could possibly get - and helped plan the next assault. It was only after the disaster of the Nirnaeth and the complete collapse of any hope of taking out the Dark Lord that he allowed the Second and Third Kinslayings.
-Maedhros, with Maglor, rescued Elrond and Elros - and kept them alive and safe throughout the War of Wrath - and, judging by the way the half-elven turned out, raised them pretty well.
And ultimately, yes, Maedhros was the one who decided to take one last stab at the Silmarils. It was a stupid idea. Maglor said it was a stupid idea, which says a lot about him. But he didn't exactly do it willingly. Maedhros' entire outlook is based on the fact that they swore not just by the Valar (who could release them from their oath) but by the Name of Iluvatar. The only other use of that Name we know of is in Elven marriage ceremonies - you know, the marriages that are unbreakable while the world lasts. To Maedhros, this wasn't just 'hey, let's get some shiny rocks' - it was 'if we do not (try to) fulfil this Oath, there is a very real chance our souls will be forfeit'.
Was he a nice person? No - he was a son of Feanor, and everything that entails. But he was the one who tried to be nice.
As to Maglor, well, he was a harper, of course - wrote a very famous lament about the Fall of the Noldor. But beyond that, he was rather weak-willed. He stood by his father when they burnt the ships, and when the High Kingship fell on his shoulders after Maedhros' capture, he didn't even try to rescue his brother. And ultimately, he was persuaded by Maedhros to go along with the final theft.
But he doesn't come across as a bad person. There's a comment somewhere that of all the Feanorians, Maglor was the most like his father - with respect to the non-insane parts (the most Feanoresque nutter was Curufin, who's actually named after him). Which means he was viewed as the successor not to the Kinslayer, but to the elf who made the Silmarils. Sort of explains why his music was so highly-regarded, yes?
And on top of all that, there's the fact that - unlike Celegorm, Caranthir, and Curufin - neither Maedhros nor Maglor ever displayed any hostility towards mortals. I'm sure they had as much disdain as you could ask for... but Caranthir left the Haladin to die in order to get an edge in negotiations, and Cel'n'Cur wanted to cut Beren loose without any aid whatsoever. Maedhros and Maglor accepted the service and fealty of Uldor and Bor, and fought alongside them in the Nirnaeth - which didn't turn out too well, all things considered, but that wasn't their fault.
And on top of all that - Maglor's the one who never went home.
--he who harps upon the far
forgotten beaches and dark shores
where western foam for ever roars:
Maglor, whose voice is like the sea.
That, ultimately, is why he's a sympathetic character - because I sympathise with him.
hS