Is it possible to edit the number of missions that spawn each month (to reduce them)
or reduce the penalty for not attending them
or increasing the reward/penalty ratio in favour of the missions I do attend
Yes, but all of these would require quite a bit of manual editing of .rul files and some thinking about what your
exact goals are.
or perhaps add an arbitrary number to the score each month to soak up some skipped missions?
Making a score-granting pop-up event would be a pretty neat way to do that with relatively little effort.
I get that these are valuable for training recruits.
In fact, it is really not possible to get the same value out of anything else short of
extensive save-editing, since a lot of your veterans' power comes from commendations, and there's no easy way to get those. Well, maybe editing the commendations and making the triggers trivial, but
that would be pretty much cheating to such a degree you could just dispense with playing the tactical part altogether.
If you can live without the commendations, using debug mode's auto-kill and auto-stun options to skip tedious ground missions is perhaps the best solution. You'll still get to see the maps, manage loot and prisoners, maybe even take a few pot shots beforehand.
I wouldn't want to attend them all though for 3 years before the invasion begins. I won't have the time to ever get that far.
2 years. And if you lack the time to play a megamod, there are others that are shorter.