@turndown4rip Interesting. I believe in your scenario combat would have to happen.
You say above that the UK would wait to declare war until that moment. I find it hard to believe that they wouldn’t already be at war though. The moment Germany attacked France, the UK would have gone to full wartime income. While true, that doesn’t necessarily mean that the UK has to actually declare war on the Axis at that moment in time. And per the rules, as you’ve surmised, they can declare war at any moment during any portion of anyone’s turn.
All that said, I’d have to imagine a German player will have already hit the UK fleet in the Atlantic somewhere, making war already a thing. But I may be getting off topic a bit, as your scenario is technically plausible!
I guess with your situation above, the UK has declared war after Germany’s combat movement phase. So no combat would happen on Germany’s turn. But when the UK’s turn came around, the UK could choose to move it’s ships out of the shared seazone and away from combat. If they left the ships in that zone though, combat would occur. You couldn’t just decide to leave your ships in a contested territory while at war and not have combat occur.
To your Dutch query, I’d have to agree with @ArtilleryAficianado. While you’re correct that those are valuable resources the Commonwealth as a whole losses out on, the Japanese would also be hurt as well by never having all that income, or the bonus income that comes with it. I think that would balance itself out that way. Plus, I think a smart Axis player would effectively negate the Allies from gaining much of that income. A smart move would be to wait to attack the Netherlands itself as Germany until Japan is simultaneously ready to take the DEI, to stop the very issue you bring up of the Commonwealth gaining that income!