overfermentation

With a long fermentation, several different things could happen (maybe more than one of them) -

1. The dough runs out of food - fermentable sugars or the ability to break down starches to produce them.  If this is the case, better not extend the fermentation time, even retarded.  It may not rise any more after being shaped into a loaf.

2. The gluten gets damaged (by proteolysis).  In this case you are probably done for, since working the dough won't be able to restore a good gluten mesh. You certainly don't want to extend the fermentation period.

3. The remaining starch has been damaged too much.

4. The yeast has gotten inactive because it has used up all the food nearby.  In this case, the dough will probably become more normal after some manipulation to move the yeast or sugars around.  You may be able to stretch/knead it and get more fermentation time.

5. The gluten has been relaxing and changing its properties but the manipulation needed to shape a loaf will restore it well enough to bake.  In this case it might work out to shape the loaf and retard that.

6. It's even possible that you could knead/stretch the dough and get enough gluten improvement that you could do some more retarded fermentation.

I think you would need to try shaping and baking a loaf to know which of these was happening.  This might help you out the next time it happens.  In general, with wheat doughs I have seen as much as tripling of volume after an overnight ferment.  I don't think more expansion would be a good idea, and if I got a tripling and still wanted a longer ferment, I would degas and stretch it.  If the dough felt springy and lively I would not hesitate to ferment it longer.  If it felt gloppy and pulled out without resistance, I'd put it into a loaf pan and hope that it could still do some rising before baking.

why does my dough always eventually fall below room temperature during the bulk phase?

That's a new one on me, and I have to ask whether you measure the dough and the room temperature with the same instrument.  Remember, just because a digital thermometer has good precision in its readout doesn't mean that it's necessarily accurate.  One way to lower the temperature is through evaporation, but it doesn't seem likely to me that after hours of bulk fermentation there could be enough evaporation going on to have a noticeable effect.