The reason why your belt "slides down" is because your belt is a crappy gun belt. If you buy an actual gun belt, you will notice a HUGE difference in carrying a gun. The belts can be expensive, but it's worth the money. I am a big fan of the Wilderness Tactical Instructor belt.
This is correct. Check out a good belt. I just picked up a Daltechforce Super Biobelt and it is amazing what a difference a good belt can make. Esrice did a nice write up on several belts including this one. Read his reviews and then look for a belt that fits the budget.
What position are you wearing the holster at? That can make a huge difference on the gun/holster being comfortable.
I'm surprised at the issue with retention. Most of the Kydex hybrids fit very well.