I don't know the legalities of septic tanks and solar(other than it is illegal to have a soakaway anymore) but the attitude of all the locals I know is "just do it but quickly". I would observe that a forca septica is underground and if it is done quickly who is to know.

Gas contracts are as previously stated. You can get one for a fee even on a caravan out here if it passes the inspection. Once you have one you can get several bottles. Or you can just buy a few bottles as know one asks for Id etc when exchanging them. It's the contract situation that has made Spain one of the few eu countries that has a gas bottle black Market. Go figure.

As for solar panels on a finca you would be "farming" sunlight? I actually know someone who used this argument on an ayuntamiento technico successfully and will happily use it myself if needed.
Bruixot