Old Instrument compatibility is mostly on Microsoft (wired stuff). The ION/Pro/midi stuff was apparently used by ~1-2% of the total user base and in order to keep cost to develop reasonable (since they self-funded with Mad Catz) they had to cut underused features. This is also why online multiplayer was dropped.
RB3 Export: Can't disagree here. If I had to guess I'd say not planning ahead and securing the rights to the songs for DLC/export was either missed or too expensive and the previous publisher didn't want to pony up the cash. This is a huge bummer for us either way.
RB4 Export: I don't know, if the intent was to make this the "platform" Rock Band was initially intended to be then an export code doesn't make any sense here. Personally I'd love it if the box came with a code to just let me download the game, but I need the Guitar bundle to get a full band up and running and I'm not double dipping until the download price drops.
Lack of Details: I think part of the problem is HMX is super busy getting the game out the door. They aren't a huge company and Nick Chester is only one man. At least some of your questions have been answered even if not in an Official FAQ. There is a single player "campaign" where you earn fans and have a manager etc. "The campaign, meanwhile, is perhaps Rock Band 4s most interesting reach in the gameplay design department. It is, effectively, a Rock Band RPG" Source
