Force Carbonation Question

You need to keep the pressure on. As the beer absorbs the gas in the pressurized headspace, the pressure drops. Thus, you've got to keep replenishing it.

A compromise is to pressurize it to, say, 50psi. Then you can disconnect the gas and let it sit. Not perfect, but it will get you in the ballpark of where you want to end up while freeing up a gas line for other uses - which, I assume, is the issue, here.

 

You mean two regulators - either a second primary or a secondary. Multiple primaries look nicer, IMO, while multiple secondaries are a bit more flexible, location-wise. A second tank is overkill simply to dispense from a second keg (you'll need a regulator for each tank anyway), but I'd strongly advise to to get one anyway as a backup. There are few things worse than running out of CO2 in the middle of a party. The high pressure regulator tells you exactly one of three things:
1. there's liquid CO2 in the tank, with zero indication of how much (could be a single drop, in which case, there's not enough for tonight's party)
2. the tank is empty (not enoiugh for tonight's party)
3. you're running on fumes (literally). IOW, the tank might as well be empty. Again, not enough for tonight's party

Notice that, in all three cases, you don't know if there's enough for tonight's party.

 

I can't be sure without checking after work, but I think all tanks have a tare (empty) weight stamped somewhere on the outside. You could weigh your tank to determine if there is 1 or 3 or 10 lbs of CO2 still left.

 

That's the simplest, most reliable, and, since you buy it by weight, the most direct way to know how much CO2 is in the tank.