OK... let's sort through this.
The 294MC sensor does not have any UV or IR filters in front of it. Therefore, it will be susceptible to those wavelengths, which will cause bloat / blooming. The reason you would use a UV/IR cut filter is to eliminate that.
A UV/IR cut filter does ABSOLUTELY NOTHING for your light pollution. The filter allows the entire spectrum (from ~400nm to ~700nm) through. This is a GOOD thing for galaxies and other broadband targets. You DON'T want to block any of the spectrum.
The LP filter strips out swaths of the spectrum in an effort to combat the artificial lights we humans developed to illuminate the night. Since the vast majority of these lights use sodium and mercury, the LP filters block those wavelengths.
The likely reason you're seeing "more noise and grain" now is because at the same gain/offsets more light is hitting the sensor using the UV/IR filter than was hitting it using the LP filter. Ultimately, the solution is to take more subs and have a lot more total integration time.
To answer your other question, no you don't stack the two filters... even if you could.