1. Compare the combo and non-combo potencies for Slug and Clean Shot. Using a shot without a proc isn't worth it. Just keep hitting 1 if you don't get a proc.

2. For Reload, yes. If Reload is used on cooldown, you get two reloads per Wildfire. Use Reload > 1 > 2 > 1 to get procs on both 2 and 3. Use Wildfire here if it's up. Then try to have Quick Reload available so you can use 3 > QR + 2 > 3 to guarantee two Clean Shots. If you you only need to delay Quick Reload a little to do this, then hold it. Otherwise its probably better to use it on cooldown too.

3. Direct Hit seems to be the best stat at the moment, but the others aren't far behind. To my knowledge, MCH does not gain damage from any stat significantly more than the others.

4. No idea. I don't think anyone has come up with a clear answer to this yet.