There is some chance that the setup routine can migrate the 1.0.1 database. I would not know for sure, but if you can back-up before trying it would be worth the effort.
In answer to your questions:
1. If you plan to do this via Zenphoto you will have to use pure SQL in accessing the old table. The zenphoto routines are wired to a single database.
2. Yes. in general objects will use the same function name if they provide the same method feature. This is more so in the next release, though, so be aware. This project will be substantial. I suggest you review the Zenphoto object model, etc. So you know how Zenphoto currently works. (Sorry, I don't know what you review to see how 1.0.1 works.)
3. In current releases the comments table contains an "owner" type and ID that link it back to the object. Don't know about 1.0.1, that is before my time. Presuming it is the same, you would have to instantiate a comment object and fill in the methods/proprieties.