Haven't tweaked the events yet for the updated max battle group damage. If you want to work on the events, just say that you have them.
Maybe I'm just a little worn out from shoveling but I don't see where I can change the defensive bonus of a river - is this a hard coded thing? I didn't see it in Cosmic and didn't check if TNO made it something that could be changed in cosmic2
I think it is hard coded. It is fairly easy to fix the problem on the railtrack terrain by simply reducing that terrain's defensive bonus to 50% from 100%. The trouble comes from other terrain types. At night, the river defense bonus makes a huge difference in how easy it is to kill urban targets. A couple lancasters can usually kill an urban target without railtrack under it, but it is much more difficult to kill targets with railtrack. This isn't necessarily a huge problem that some targets are softer than others, but it means balance might have to be checked a little.
The more serious problem is that fighting on the ground is impacted, particularly fighting between battlegroups, and only because battle group battles have such high stakes, since there are so few of them, especially for the Western Allies. Maybe when a battle group is defeated, a counter is started to recruit a replacement in 10 turns, but only one battle group can be recruited at a time (so significant losses hurt, but modest ones can be recovered from).
We can use lua to override the battle calculation, and substitute in our own if we like.
Increasing the maximum munition damage would increase the advantage of air power, which would favour the Western Allies in combat.
I did change the rules to reduce the gun battery to 30 defense from 50. I went with 30 because I'm pretty sure they'd be one shot by most of your Tempests if they were lower than that, but we'll see.
Maybe make it 10, and give it x2 vs Air, which I think actually means x3 vs Air and x5 vs missiles. That would protect it from bombardment, but mean a battle group can rush in and defeat it.