Haha well, while I've finished XCOM: UFO Defense many many times, the last time I finished a game of TFTD was in 2015, so I did not remember it. Sorry, Solarius!
Look under alienDeployments, there is a definition like "nextStage" or something.
You can use the standard Mars terrain, you only need to change the mapscript.
Yes! That's exactly the script. I even can define a winCutscene.
3/ addUFO command in map script
I was reading a very useful page of the UFOpaedia (I even printed it,
caray!), on the Ruleset Reference and in the "Map Command" section it's what you talk about, Meridian. I imagine that the "addUFO" command to add an Avenger, it must be defined as
UFOName: STR_AVENGER, is it correct?