Typically, yes, one does play both the human and Digimon characters of a pair. It tends to work out as not being "RPing with yourself" because the Digimon characters do all the actual fighting, and the human character takes either a strategist or moral support position, whereas outside of combat, the Digimon knows little about real world things, and the humans won't know much about Digital World things. Then again, I'm not sure what your idea of that actually is. (Then again, you rarely run into a shortage of material near the beginning, because you're already establishing a team.)
I'd allow it if someone would be willing to play half with you, but typically the interaction between the Humans and their respective Digimon is key, to the point where it is actually the major defining factor of the Digimon's various forms. That being the case, I can see a lot of people not wanting that to be left to chance. Furthermore, it sort of leaves one of the two involves stranded should the other need to take a leave for a time, vanish, or drop out of the roleplay. See, if one person drops, and the human and Digimon go with them, then we can typically find a way to write around it or induct someone else, but you can probably see how it might be difficult if the pair is split, so it's your call.
Anyway, about plot three, I realized I didn't word something correctly, or rather omitted something that I added near the end. The "restrictions" for that particular plot mainly involve the characters being Japanese school age kids living somewhat close to each-other, simply for the convenience of coordinating events in the Real World, characters not having military parents, and the Digimon not being given a backstory that puts them chronologically before their generation, which basically means the Digimon's whole life is within the rp, and only their personality is defined beforehand. That's not really unheard of for Digimon.
However, you may, alternately, show the most interest in plot 4b, in which, while I didn't state it, I'd probably play the "base" stage of the Digimon, if not all stages, but I can see how that would either make some people feel cut out, or bog down the progression by separating the posting of human and Digimon.
[@Forsythe]