This is strange. By August '98, you've had 11 occasions for the manor-generating script to run, with 32x3% chance each time. So the expected number of manors ought to be 11*32*0.03 or near enough 11. That is, one per month. My own experience tends to reflect the roughly one per month nature of manors.
There's of course the tiny chance that you get 30 manors per month, but it shouldn't happen
all the time.
AFAIK, difficulty should not affect mission generation, so Superhuman
should not matter. Perhaps I'm wrong?
'Many of them have MiGs' is probably the fact that manors are coordinated to
all stay roughly at the same tier, and so upgrade every 8 months on average. Although you should be seeing other fighter jets as well, unless your Earth has been taken over solely by Red Dawn.
...surely it is not intended to have this amount of Manors while closing 90% of all cultists cases.
You obviously don't know Solarius.
Migs of your own are gated behind promo 3 and several research projects, so the absolute earliest you can get this is by late 1998 and even then, you would go equal.
Eh, depends on the weapons. Avalanches are an auto-win button. Stingrays are a crapshoot, true.
Theoretically, you
should be getting to Interceptors by the time the invasion starts, and those are pretty good against cultist craft.
Another issue (and correct me if I'm wrong) is that the Manors seem to generate negative score every month?
Yep, all 'alien' bases do. Though the worst you get from even the biggest manor is 5*30 = 150 a month. 20 of them would be pretty bad, though.
Edit: The bigger problem is that high-tier manors can upgrade as well, and those missions are much harder. Hybrids, Golden Academy, extra High Temples.