Can't answer for the quality of the Rollei filters, but I know that if you want to buy modern filters for your TLR, you can buy one of those Bay I - 49mm adapters. I just bought one from Camera Hunter on that auction site that had a power outage the other day, and it's pretty useful.
I think one of the issue with old filters is that they may not be coated against reflections. Modern ones like Hoya or B+W will have this protection. Regarding q.4 I would be tempted to say yes, because it means some spots will receive more intensity than others, but you can only find out by trying it.