[Hey, don't worry about it, I'm mostly just a lurker too.]
Well, I figure there's an infinite number of entrances to anywhere since it's Touhou we're talking about, since you can make a portal (or gap) to it if you really wanted to. :V Granted, most people probably can't make portals, but... Anyway, I'm just saying that the heroine in games like TH1 and 5 seems to go through portals other people already made (except if you go to old hell in TH1, then she probably just goes down a hole...). Or at least, that's my theory.
Also, I haven't thought too much about what part of Makai that is in the first game. I drew some parallels between the final battles with Shinki (TH5) and Sariel (TH1) though: Six wings, both eventually get serious and change form to some extent (Sariel's background music even changes when she gets serious, I don't think anyone else in Touhou can say that, though I don't really know), background changes as the battle goes on, etc. So I like to think that Sariel has some sort of relation to Shinki in some way, though I haven't any idea what that is (well I do have quite a bit more speculation actually, but I'd better stop before I start spouting too much crack).
Ahem, sorry for the off-topic (well, I don't think it was completely off topic though...).
As for Mima getting into and taking over Reimaden without Shinki noticing... I think that if it was an old, abandoned structure she might not notice.
However, Shinki did say in Mystic Square that she "created" Makai. We know she gave life to its inhabitants, but that's not to say she didn't also create the dimension itself (if it is a dimension and not just a remote part of Gensokyo, but I like the theory it's an actual dimension so :V ). Anyway, if you created your own dimension and not just a dream world, I'd like to think you'd know if someone entered or left it, and where they are at any given moment, as well as perhaps other details if you so desired.
You have a good point about Genji's dialogue in TH2. It really doesn't look like he would mention them separately like that otherwise... I think. Well, if Reimaden is a specific place in Makai, perhaps this was the closest portal available to it. The shortest route. I dunno.
[lol mushrooms]
Erm, I dunno about the name of Gengetsu's world, but... I was reading one of Ruro's fics and she called it "Tsukuyomi Castle" though I dunno if she made that up or not. :getdown:
Eh, the PC-98 games are all about dreams? Hm, I'm not quite sure what you mean by that... Sorry, I'm newer to the fandom than most.
I probably should not comment on Shinki's or Mima's power levels explicitly, since that might be frowned upon, but... Let me just say this: No matter what Mima says, you can't tell if it's the truth or not. In TH2, she said she would go all out the first time. Then after the heroine managed to push her back anyway, she said she was going to go all out again and used more power. At what point does Mima actually become serious? Well, we know how much of a troll she is. Just look at her dialogue in Mystic Square.
And even then, she was trying not to kill the heroine (she said it herself, she needed Reimu alive). Not to mention that Genji himself seemed to sense something about Mima and warned Reimu over and over that she'd surely be no match for Mima.
Also, as said, the fandom seems mostly convinced that Mima is Phantasm-tier (just look at Twilight Spark). Well, I don't think that's without reason though.
Well, this is just me though. I consider Mima to be a master magician, perhaps the best magician in Gensokyo.
Midboss is Mai? Hrm... The colors don't really match up, I don't think, but hey. Maybe Shinki was already getting annoyed about this intruder that kept killing whoever she sent, so she eventually sent one of her more powerful minions (Mai) and the heroine ended up taking care of Mai just cause she got in the way. That's so totally like Reimu. :V
Anyway, it seems that the reason Mima was in Reimaden in Touhou 2 was one of two things:
1. It was her home.
2. It was somehow related to the "restoration" process she wanted to undergo. In this case, maybe Reimaden has something to do with the Hakurei orbs... Somehow. Seems really odd to think about it this way, but...