If you mouse over a nation's flag in the Diplomatic triangle, there's a tooltip to inform you if a nation is closely enough aligned. only the faction leader benefits from other nations' strategic resources.
As for submarine fleet sizes I usually group them in threes. Eventually, they will get found. they will get engaged by enemy planes or ships. at least this way there is a chance that some of them might get away. I'll test running them solo or in groups of 2, though. never too late to learn new tricks, is it?
The real trick against Poland is kind of gamey. If they are not mobilised, there is no need to mobilise the German army either. your weapons and doctrines are more advanced than theirs. Even unmobilised they are no match. just watch the French, though. your border divisions won't be mobilised either. And trust me, the French WILL mobilise ahead of time 99% of the time.