I guess you will soon get a comprehensive answer from the Telerik Support
In the meanwhile, you could have a look at this topic concerning RIA Services support in OA.
As of March 2010, I guess this was still work in progress.
Few elements of answer :
1 - Yes, definitely, classes are partial from the start, and fully inheritable.
OA Separates generated code from your custom core classes (accessors and other stuff) by using partialable classes (optional, but very handy).This is a very clean and clean feature that is customizable.
2 - No idea, to be honest, but I guess it's feasible.
3 - Not to my knowledge. You have to handle this in the code with eventHandlers, i guess (I may be wring on this one).
4 - Nullable types are put by default on nullable columns.(i.e appending a question mark sign to .net types to make them nullable).
If your column is non-nullable you can "fake it" in the mapping. I had the same issue and it can be managed by editing and "patching" the mapping files to force nullability, since it is all a question of mapping after all, if you Database intercepts smoothly such queries with no sql exceptions (via a trigger, like you said, for instance)
All in all Telerik's OA is an excellent product and I generally strongly advise it.
However, you should think twice since you seem to have a very specific need.
Hope this helped.