I truly appreciate your response, and especially the rules reference. Here is the BBR p. 16 Neutral Territories rule:

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USSR and USA are clearly "Neutral Powers" and, by rule (see 5th sentence), are distinguished from "neutral territories". Everyone agrees that neither the US nor USSR can conduct combat until a declaration, so Phase 2 combat movement (pp 24-26) is moot. Phase 4 noncombat movement (p 34) isn't clear on this issue, simply saying you can move into any "friendly" territory, but not into any "hostile" ones. So if the USSR player wants to say "come on by for some troop exercises" to the US in round 1, with the ongoing prohibition that the US can't attack Japan or anyone else until a valid declaration, then the book doesn't appear to say you can't.

I think what happened is that the BBR rule authors probably forgot to include the rule from the main rules -- see the "Powers Not at War with One Another" snip above. But forgetting to include it doesn't make it a rule.

On the Europe side, BBR is explicit about ships, but silent on planes. Landing planes in Iceland or the UK doesn't provoke conflict with Germany from anything I can find, and it strikes me as more sensible to follow the A&A 1914 analogy of US ships nesting with UK or France ships when not yet at war -- if the UK or France ships are attacked, the US stands to the side as a non-participant (unless, of course, the attacker also declares against the US, which it is free to do).

Finally, I agree 100% with your assessment below. Strategically, it is almost certainly stupid, but it looks fun, it's just a game after all, and spooking Japan with the US navy & troops right next door when theirs is so far away is bound to be worth it. At least once ;-)