Well, for the edification of others, here's my own answers.
1. Pollution happiness is in Govern.txt. The Pollution tolerance modifier relates to the number of p.u. it takes to cause -1 unhappiness.
2. This is triggered by #4.
3. No, except for the cryptic smokestack indicator in the upper right corner. Still haven't figured out how useful that thing is.
4. Const.txt has values near the bottom of the file for the 'dead tile from pollution' chance and threshold. The threshold is set to 400 (answer to #2). The chance is .005 - you have to be substantially over the limit for quite some time to get a dead tile, but since you can't fix such tiles until late in the game, the damage can be huge. Both can be easily changed. (Global Disasters are stored in pollution.txt)
I prefer to play with more unhappiness in the game. (perverse, huh?) Accordingly, I lowered the limits at which pollution kicks in (300 production, size 7), but then raised the limits at which it causes environmental damage. There is another limit on total unhappiness from pollution, -10 I think.