Would it be possible to make an abillity which cost money (and a good deal of time I might add) but spawned ships in its immediate location?
Making an ability that spawns ships is pretty easy to do. You just need to change the
AbilityDeployCombatTurret.entity to your liking. Making it costing credits is not possible though. (At least currently not.)
If I can do this it would be interesting if I could make these so-spawned fleets essentially act as fighters of their parent capital, and therefore could only travel with the capital ship (maintaining my desire to make them the focal point of a fleet unit)
For this you would better modify the fighters/bombers of capitals (or add new ones). Then those ships will only be able to travel with the capital. But then you can't let them spawn by an ability.
Is it possible to make an abillity that disables or prevents the use of a specific other ability (so that any enemy defenses in a system will prevent the 'reinforcement' ability noted above!)
Well there is a buff that disables abilities but you can't choose specific abilities. All abilities get disabled.
There's also the buff that prevents fighters launching, if you would want to have your spawned ships as fighters/bombers type.
So a lot is possible, just experiment with the files.