In fairness, an argument could be made that I didn't help matters by moving a number of units to the west rather than having them dig in along the coast. I have considered it. There are two shortcomings to that strategy which I see. Firstly when you have a large number of units in a small area their utility diminishes because of crowding, particularly if they're all infantry. You end up with a lot of units just sitting there unable to attack because they're too far behind the front lines, even with the mortar attack by Polish heavy infantry. Secondly had I not moved west it would have allowed the German units there to effectively flank me, if anything hastening the collapse of my forces. I actually think I'm doing fairly well in that particular front, or at least I was until my supply went kaput
I never did get a reply to my question about Poland having a sub. It was present in the single player campaign after all. And although a lot of the polish air force ultimately relocated to the UK, presumeably this didn't occur until at least a bit of fighting, so I do think a fighter or two wouldn't be that implausible.
The best way to get a feel for a map is to play as both sides of course, that's why I'm so confident the Korea scenarios are already very well made with minimal tweaking required. So far I've played two games of the Polish invasion, both as Poland, so hopefully at some point I can try things from the other side and see how it goes.