Ok, so you have tons of questions.. I will start from the bottom..
3.X It works in the way you want out of the box.
2. you never need to fix that table, never touch that table because it contains internal information from the engine, users should never touch that.
1. It depends on what you are doing with the session. If you create one session per process it will work. if you have concurrent threads trying to modify a field in a database one of them will fail and the other will win.. that will happen in any database and not only in jBPM. For that reason you are seeing errors in the console, but the state keeps consistent with the executed code. What you can do is to implement a retry mechanisms if one call fails because another concurrent one was processed at that time. If you want more specific answers on these topics I think that you should contact us for consulting services..
Cheers