IIRC...
1. You can have as many domes as you like on a province.
2. Only 1 dome of each type will be effective.
3. Extra N and A domes are still useful because you have a fail-over capacity for broken domes.
4. I don't know what determines the order they activate in, sadly.
I don't think anyone does.. but we could probably determine via test.
Ie., some domes are brought down by things, forest dome by fire...
it would be interesting to find out if the domes were done by ongoeing spell effect number, alphabetically, or what ahve you.