As long as your nute temps are 58-70 your ok....50-55 won't really hurt anything but growth rate. Flush them out with PHed water...I would say run it with just water for 1 day then add the nutes and such.

As far as ambient temps Stinky is right but keep that res under 70. I second the idea of a foliar spray...iv used a very dilute spray of micro nutes in the GH 3 part with no trouble.

I dont think you will see an improvement in the old growth...not sure but keep an eye on the new for sure. The spray should help. Since the res is so cold you could run the temps in the 80s no problem. That should help.

Go buy some insulation and wrap the buckets up...much more stable that way. Easier to control the temps, you could even add some warmer water and with the insulation the temps should stay much longer.

I wouldnt say im an expert but I do have some experience....I would feel more comfortable if an expert chimed in..but I do feel pretty confident in my solution.