2. I don't know about the P30 but I know HK has a factory threaded barrel for HK45. I don't think it would be too much to assume they may have one for P30 as well (not sure about P30L at all)
3. Heinie makes raised sights for P30, not sure about Mepro or Trijicon though.
4. No...sorry. Some could be fabricated with a bit of time, money, and someone good at welding.
The HK45T does exist, just not as a gun shop option (as far as I know). The threaded barrels are available from HK though.