Autosave is disabled by simply going into the save tab of options while you are in a game and unchecking the autosave button.

I have no idea how the Galaxy Forge problem works, as I have no experience with it...

Well, DLed your map, dropped it in the Galaxy Forge folder, and it seems to be working fine.  I did change the occupied world from being already owned, though, thought that might have been the issue as it did seem a tad odd.  But switched that back just to double check, and the map loaded up just fine (even got the space ponies achievement out of it.  Heh.) Also ran with all victory conditions off, and on, no issues.

So, not sure what's causing your auto-defeat, unless you just need to move it to the Galaxy Forge folder in \My Documents\My Games\Ironclad Games\Sins of a Solar Empire Rebellion\ instead of MapDesigner (that folder is intended for those designed in game, not through Galaxy Forge).

The map does, however, have an amusing.. 'issue', in that it forces me to be TEC Loyalists regardless of which faction I choose (and I just left it on random, honestly.)  Which is kind of nice to know for map makers, should they want specific factions to start in specific places on maps.

Here's a hint when making new maps,  I always use the in-game feature for making my maps. then use the forge to edit and move things around like I want them.  this is faster and it helps to keep you from those issues you are experiencing.

Just some FYI!