I did a similar thing a few months ago, but I fermented it in a single batch, then split into 5 1-gallon containers to secondary and dry hop. Not too much work and still got the result I wanted. I'm considering doing it again with 5 different hops.
I like this one a lot:
http://www.thermoworks.com/products/low_cost/rt301wa.html
Much cheaper than thermapen, and the only thing you lose is a tiny bit (like 0.2F for the normal temp range you would use in brewing) accuracy, and it is a few seconds slower. And the water proofness is pretty solid.
Except that it doesn't if your fermentor leaks.
Even if both the above premises were true, and you had proved that fermentation -> airlock movement, than it still doesn't mean that airlock movement -> fermentation
I do agree that others' assertion that "airlock movement isn't an indicator of...
What style of beer is it? Extract or all grain?
I might just throw in my flavor/aroma hops now and be done. It would be less bitter than with a full 60 min boil, but shouldn't suffer any other ill effects.
Unless... It is an all grain beer with lots of pilsner malt in it. Then you would...
This should be simple. Most PIDs have a built in relay that can switch a few amps, so you would only need to add the 3 way switch.
However, why do it this way? Why not do it the Kal way and set your HLT temp to the mash temp and recirculate the whole time?
Did you use DME for priming? I've heard that this can cause a krausen ring in the bottle, but it wouldn't really make sense for only a single bottle to be affected.
Could be an infection, if by "less beer-like" you mean maybe lighter bodied.
If this is something that really keeps you awake at night with worry, I would just buy two temp controllers. Wire them in series so they both have to be closed for the fridge to kick on.
If they both fail closed at the same time, don't worry about your beer because you are also about to be hit...
Definitely do 10/4 cable (usually labeled as 10/3 with ground). The price difference is almost non-existent, and then you'll have that extra conductor there if you decide you want it.