1. Generally speaking, yes. As Shroud mentioned, pro charting almost always requires a little fudging anyway (no left-hand on the keyboard, no string bends on the guitar, only three toms on the drums and so on), but other than those necessary compromises, the pro instruments should be as realistic as possible. At the bare minimum, the rhythms should be accurate, toms and cymbals on pro drums should align with where toms and cymbals are heard in the song (even if they are different toms or cymbals), and pitches played on pro keys and pro strongs should match pitches heard heard in the song (if perhaps in a different octave).
Some charters have copied 5-lane keys to pro keys for the sake of having "fake" pro keys, but this is a compromise to make Magma happy and not really something people are doing just to pull a fast one. In reality those charters opted not to chart pro keys at all.
2. Of course. Charters are always welcome to ask for help here. Sometimes it seems the old vets are quick to bite the heads off newbies, but really that's only when someone asks a basic question covered by the required reading. If you've done your homework and you still find yourself stuck, people will be happy to lend a hand.
3. Only rarely does a song have a single, steady BPM, and it almost never happens on songs released before 1990. If you're convinced your song has a single BPM through-out, Shroud's suggestion of using REAPER's "tap" feature will help, but really you should be prepared to tempo-map your songs to ensure you really do have all the beats properly aligned.
4. What Shroud said. We do have a lot of different language speakers here, even one or two that have dabbled in Japanese, but I don't know that we have any actual native Japanese speakers. Other forums can help.
5. First and foremost, I and most authors agree with you. If the original song has a part, you should be able to play it! However, there are several reasons why a particular submission is missing instruments. Keys are especially notorious for being buried in the mix, and unless you have a good ear and/or a firm grasp of music theory, it'll be extremely difficult and painful to create a playable chart. Other times, a charter is simply not interested in playing instruments beyond what they've already charted, but still felt like submitting their stuff anyway, just to share or perhaps in the hope that others would pick up the remaining instruments. In any case, most authors will be happy to hear you've taken an interest in improving their stuff. Like Shroud said, ask nicely!
6. An exhaustive list isn't really possible, but generally speaking, here is what you need to know.
- Game controllers intended for the Rock Band or Guitar Hero series on your console will work with Rock Band 3. This includes all the guitars, all the drum kits, and of course the RB3 keyboard.
- Most USB microphones will work, including those bundled with games for other consoles, and even mics sold separately and not intended for music games.
- 99.99% of all MIDI-enabled keyboards will work when using the MPA. In fact, I'd be surprised to hear of a keyboard that doesn't work. Keyboards with the older 5-pin MIDI ports will work out of the box, while keyboards that support only MIDI-over-USB will need a little more finagling to work with the game. Such finagling is documented elsewhere here.
- Most MIDI-enabled electronic drum kits will work when using the MPA. If your kit lets you change the MIDI note(s) for each pad and cymbal, you can make it work. If your kit can't be reconfigured but claims to support the "General MIDI" standard for MIDI drum mapping, odds are it'll still work. Again, drum kits with 5-pin MIDI ports will work as is while drum kits with MIDI-over-USB will need a little extra help.
- Officially, only the Fender Squier released specifically for Rock Band 3 will work with the MPA for playing pro guitar. The Mustang game controller, the one with 17 frets and 105 fret buttons, can also work with the MPA, which can be handy when you have a Mustang for one console, but the game for a different console. (The RB3 keyboard can jump consoles with an MPA too.) Unofficially, there is a discontinued electronic guitar called the You Rock Guitar that can also be made to work with the MPA to play pro guitar. To my knowledge no one has successfully gotten any other electronic guitars to work with the game.