Synergy skills

I've recently capped out synergy @80 and am a little curious what advice people can give about the skills you can learn.

-Feed fewell - I currently only have this capped out at 5 because I thought it would help add fewell quality.

-Thwack furnace - I understand this can stop an explosion and I have so a few times. I was wondering what exactly each level added, weather it be increased % to stop explosion, prevented less durability drain or what not.

-Operate pressure handle/Operate safety lever - From what I can guess these 2 kind of go hand in hand. Safety lever can reduce the time between explosions (I think). I'm total guessing about the pressure handle, but does it reduce the durability loss after explosion, or does that only affect damage taken.

-Repair furnace - Does each level increase the amount restored?

Pressure Handle lowers the pressure. Lower pressure means better chance at better results and less damage on explosions. (Not 100% sure about it, but that's how I see it.)
Safety Lever removes impurities. Overloading can only occur at or above 35% impurity, I believe, so lowering it when it gets close prevents overloads.

Repair Furnace indeed increases the health of the furnace more with each level, but you rarely need it.

Recycle is pretty much useless, since you cap cinders out very quickly, and the only thing to spend cinders on is carving evolith slots in low level gear.