Gas leaks and tanking gas:
Got it to work with a little research. FTB wiki gave me the point I was missing. I used the air pump hand cranked (by me) in the gas cave, with a liquid tank beside it. When I first tried it failed, but I was at the bottom of my 1x2 shaft and the walls were close. The pump needs room to work. A shift right-click will show green particles when it's positioned well. Once I moved it out into the center of the cave (gagging and wheezing the whole time, lots of debuffs) and spun the pump wheel the tank went to half full almost instantly. A couple more spins and it was full. So I filled the other 4 empty tanks I had on me too.
My residual concern is it's been 10-15 minutes and I am still showing a poison debuff. My hearts are yellow-green, but not seeing any damage ticks. May have to poison myself to see if that clears it. Have some rotten flesh here somewhere...
Now to figure out how I want to use this gas.
Oh, you probably want to avoid engines and exposed sparks down there. I lit no torch and stuck to hand-cranking. I didn't not try to trigger an explosion, but everywhere I researched mentioned torches will set off the gas, and some also suggested redstone would, or engines.
Poison gone. Either the client relaunch did it, or the debuffs from the diamond dolly with a snowman spawner on it did. In either case, not an issue as long as there's some way it gets removed.