To answer this question myself, after some chatting:
As long as a room is claimed or reserved at time of reset, it will reset to full 3000. If claim or reserve is lost at any point will be set to Math.min(currentEnergy, 1500).
I have done extensive math (and some discussions) and the idea of claiming multiple rooms just enough to make gains from this knowledge is not really feasible. Mostly owed to the average distance between rooms of 50, the mining time needed even with big miners and the cumulative reserve need for every additional room you do like this, not to mention the ridiculous uninterruptable timings needed to pull it off and short lifespan on a claim creep.
Put simply, no configuration like this came out any more profitable than just straight up claiming.