1. Is it possible to create an Unit that uses the option in the editor below the Special Actions tab "Finish Improvements", that AI knows how to use it?
In the unpatched epic-game*, on the rare occasion that an AI actually gets an MGL (and successfully evacuates it to the nearest town, if acquired on the battlefield) or SGL, this is exactly what it already does, although the project it will rush may not always be optimal.
*In the
@Flintlock'd .exe, the AI's MGL-routine has been vastly improved. It will now correctly evacuate a new MGL to the nearest town, and then evaluate the best usage for it, prioritising rushing a large/expensive build such as a G/S Wonder, or Factory (if such a build is in progress but still a long way from completion, and/or its Army-limit has already been reached), otherwise it will create an Army. And then it will load (and use!) that Army reasonably effectively.
2. If the editor specifies that normal Improvement type B needs 3 Improvements type A, and this condition is satisfied through 3 cities that have improvement A. Does this mean that the next city (number 4) can immediately build Improvement B without having to build first Improvement A. (For reference, I’m not talking about GW and SW)
Yes, (AFAIK) that is correct.
3. If marked for Aqueduct Allows City Size 2 and Must be Near River. Does this mean that cities on the river cannot achieve size 2 without Aqueduct?
No! Cities which are already directly adjacent to a river (or lake) have
always been able to grow to Size2 (i.e. Pop7–12 in the epic-game) without an "Allows Size 2"-building!
The "Must be near a river [/freshwater]" flags are actually a little too brief for full clarity (IMHO).
They should be understood to mean "Requires
at least one FatCross-tile with a river [/freshwater source; i.e. the latter flag is a less specific requirement] along
at least one edge": i.e. if you set the Aqueduct to require a river (or freshwater source), it should be available to build in
any town which is up to 2 tiles away from any river (or freshwater source)*. But any town beyond that distance will be stuck at Size1 forever — or until a building becomes available later which has the "Allows Size 2" flag but does
not require a river (e.g. in the epic-game, you could give the Hospital both the "Allows Size 2"
and the "Allows Size 3" flags, which would ensure that towns 3 or more tiles from rivers would stay small until Sanitation is learned during the Industrial Age).
*Caveat: I'm not sure whether that extended definition also includes the outer edges of the 'corner-tiles' (the 4 tiles which are subtracted from the 5*5 block around a city, to make the 21-tile FatCross).