I just installed my 1100s with the check valves in the rear compartments but I didn't buy any additional ballast piping so I can only reach the rear vent on the bag and not the front. Is it necessary to reach the front vent location?
Nope, you should be ok.
Has anyone put a T in the center tank to IBS line in order to put a fill/drain on both legs of the IBS?
I just struggle to see how all the water on the opposite side of the IBS is going to drain to the one leg.
You can try, but it will require you to drill a hole in the floor of that compartment. There are dozens of us that have done it off 1 leg without issue.
What did you ever decide about the check valve and the hard tank situation?
Once the IBS was fully drained, I would give the "AIR" fitting on the top center to relieve any vacuum pressure & finish draining the hard tank.