1) You cant change specific tiles you work around your cities radii (because you work them all), but that doesnt devalue tile imps. As long as you place them at the right time and of course commit to generating PW at the right time.
2) Im certain pollution overflow levels have been altered in most mods, and CtP2 is not about pumping out production and gold at least, although commerce is most important. But doesnt it make complete sense that the handicap you make by moving your sliders should effect all your cities? At least theres no out right specilist cities anymore.
3) You can raze cities in most CtP2 mods now.
4) Hmm i never saw any obvious cheating by the AI, other than the numbers in diffDB.txt which define the human handicap for each difficulty level, there really isnt anything radical to help the AI. For example, seeing the whole map.
5) That hasnt been added/fixed in the diplomacy system AFAIK.
Theres actually a thread topped in this forum for CtP2 Mod n00bers but its quite new so i dont know if it covered some of the stuff you mentioned. As always though if you see something you want in a mod (no matter how complex) im sure someone will help.