What will you do in the summer, when you do not want the heat?
"wire three 22 amp hot water dump loads in series". I trust you mean parallel.
I believe dump loads in battery systems should be DC.
I have done some AC pre-dump-load circuits with a very wide hysteresis (nearly an LVD but set to a higher voltage) operating AC loads, but they were fairly unusual circumstances.
For you, it would require a second inverter with a huge rating. Not sure it would be cost effective.
If the AC load's inverter blows up or something, the system is stuck without a load or way to do a temporary load. Would basically have to turn on all the lights, TV, wash a load of laundry, and anything else to use up power with the primary inverter...
If a DC controller blows up, can do some temporary hard wiring, baby sitting, and hope for the best.
The 70% probably depends on the manufacturer. I would tend to agree with it, but consider 70% a bit conservative.
I am not intimately familiar with the giant controllers in parallel.
The TS-60 can be paralleled, and should operate and react fast enough to keep things under control.
I figure you saw some of the problems related to delayed reaction times in C60s, and 125A going into 660AH of fully charged batteries could have some serious voltage swings.
G-