You mentioned seeing little to no fermentation activity? How are you gauging this? Is the beer in a see-through fermentation vessel? If it is in an opaque vessel and you can't see it, don't count on airlock bubbling to be an accurate representation of what's going on in there. If anything, I...
Here is a good link that tells us we can expect to serve (but not force carbonate) roughly 15-22 5 gallon kegs of beer from a 5 pound tank. That seems a little high based on my experience.
If you add force carbing into the equation, that number will dip significantly.
The amount of CO2 used to...