There's really two isses with the whole balancing act. One issue is how to correct for when a gross imbalance is occuring. The idea of "front line" bases taking time/supply before they can handle latewar planes could work - or at least help. I'd add the exception that recapturing fields that were yours at the start of the reset should be exempt (in other words, have the system try to balance things back to the original borders).
As for detecting when this are off-skew, there are three datapoints which are actually available to build that decision calculation:
- Numbers of players per side
- Number of bases per side
- Number of enemy planes facing each side
That last one is the one we're missing now. Basically for every sector your country controls or has planes in, you add up the number of enemy planes in that sector - and then total it up. For instance, if the odds are 100:100:120, but the first two countries are using 75% of their planes against the 3rd, then the "numbers being faced" would look something like 85:85:150. That ratio is greatly different that the player odds, so something is imbalanced. Meaning even though the 3rd country has a slight edge, they are in fact outnumbered.
If you combine that last number with "bases held", a picture of which side is "losing" gets much easier to discern.
Note that this would also allow the host to detect a "blitz" situation. Take the "joint ops" case of 100:100:150 odds ... but the 3rd country decides to use 2/3 of their planes to attack the 1st in a "blitz", and the 2nd country holds even coverage. Counrty 1 has 3/4 of their planes in action against country 3 because of the blitz. In this case, the "players faced" number is 150:75:125.
Here again, it's easy to detect a gross imbalance. The host could then make adjustments in favor of country 1.
What those adjustments are ... well ... who knows?