You can change your class and appearance when importing a save. As for bonuses, I don't see why changing classes would affect that in any way, so I think you're safe.

The bonuses come whether you change your class or not, so I don't think they'll be class-specific. They'll probably have to do with experience, accuracy or something like that.

You can completely remake your Shepard when importing. You will receive bonuses based on your level, Paragon/Renegade points, and the credits you had.

I hope the high level import doesn't make the game too easy otherwise it might be more challenging to start from scratch.

ME2's adept is awesome. dont abandon it just because it can't use shotguns/sniper/assault rifles... a bit later in the game you can pick one bonus weapon type to be able to use
Also, i find the SMG is a nice substitute for the assault rifle in terms of feel, though the range/accuracy is more lacking (though biotic range is really far; warp to soften them up and then unleash hell via shockwave, pull, lift, etc etc)

You can pick a different weapon "Advanced Specialization" as well mid-game. So it doens't matter.