1. You can't. It's hardcoded I think. I don't know if anyone ever managed to get walls "destructible" when you own the city.
2. Yes. Open export_descr_buildings.txt and add parthia to the faction requirement of building highways. That's the simplest but I don't know if you need to deal with UI card as well, to show proper building card on your UI.
3. If the skins you need are already in the game then it's not hard. Just make new entries in descr_models_battle.txt for the soldier models you want to use, then edit export_descr_units.txt to use the models.
4. Spread the fleet. Some of them may not get the plague. Let the affected one rest somewhere to recover.
5. You can't. But modding their appearance to 200 (in descr_strat.txt in the campaign folder) is the rarest you can get.
6. What do you mean by 'restricted'? Not restricted means the camera can go beyond your army.