Reading missions to fandoms outside your knowledge is an interesting experience. Some fandoms and authors tend to assume that you, the reader, are fairly familiar with canon, and will not have their agents re-inventing the wheel to explain canon problems. That kind of thing is most likely to be found, in my experience, in fandoms like Lord of the Rings and Harry Potter that are or have been very popular.
Canons that are currently very rare usually go to lengths to explain the canon issues. This is usually accomplished by one or the other of the agents being unfamiliar with the canon, and therefore standing in for the audience and giving the agent who does know an excuse to engage in what would otherwise be As You Know Bob conversations.
I do have a bit of an unusual perspective on this, I think, because I read the Merlin mission that Trojie and Pads did before I had heard of the show. They explained it enough that I understood why the charges were charges, but after I had watched the show (And loved it. Everyone should watch it!) I re-read the mission, and I did get more out of it. Just knowing what the characters looked like helped me a lot, and there were subtle things that knowing the character gives you that written explanations just can't cover.
So yeah, I did get more out of the mission once I knew the canon, but the mission was thoroughly enjoyable without that extra layer of understanding.
I have to hope that people are willing to read less well known or popular (within the PPC) fandoms. While there is now a small contingent of people that know Stargate Atlantis, Stargate SG-1, and NCIS those were all canons with no or only a very few missions in them when I joined. Sherlock Holmes had none (two old missions were later recovered, but they are for a sub-canon of Sherlock Holmes). Merlin had the one mission, Psych (I'll finish it eventually!) still has none, and there is only one other person in the PPC that I know of that knows and likes Sanctuary.
As for the practice missions, I point you back to July's response, again. It was well thought out and sound advice.
I was also worried about humor when I joined the PPC, and I have to admit that my results have been mixed at times. I think the one thing that writing for the PPC has taught me above anything else is to relax. Not relax in SPaG, which I think I am slowly getting better at, but with the process and characters. I've had problems in original fiction and fanfiction writing of being so afraid to mess things up, that I end up paralyzed and never get anything finished. I wish I had learned some kind of advice that I could share with you about relaxing while writing, but though I've consistently managed it with my PPC agents, I still stress over the others. If I ever figure it out, I'll let you know, because most of the best things I've come up with in missions were a result of having no plan whatsoever and writing myself into a corner that I then had to write myself out of, and I wish I could bring that to my other writings.
Anyway, I'm done with the rambling. Good luck!