You can't rob Peter, Paul and Mary to pay yourself.
A question about the support given by troops and fortifications in adjacent conquered hexes: Can they support multiple attacks on the same turn? Or only one, and then you have to decide which attack they'll support?
Yes, troops and fortifications can support multiple attacks on the same turn. There is nothing against that in the rules. You don't have to keep track which troops have given their support this turn and which have not (that would be a hassle...). Also, troops and fortifications can give support on the same turn they were put on the board.
A well-placed fortress can support multiple conquests and make you rich in just one turn