If I go back and replay ME2 with my character from ME1, will that "overwrite" the decisions I've made in my first play if do things differently? Does anyone know how the ME3 character import process will work?

As long as your saves are different, then those are the choices you will have made. You can have multiple iterations of your imported ME1 char and whichever ME2 save you load into ME3 will determine the choices made.

That's how it was from me1 to me2 at least

No it'll just give you multiple save files to pick from when you choose to import into ME3

I assume the same as the second. You'd just get another Shepard (with the same name) on your list of importable characters

It's been so long since I've played ME2 that I don't recall the import process. I definitely want to go my first playthrough but want to explore other things...

...like shacking up with Jack.