I believe you are correct. The “bonus” 30 the US gets when it’s attacked early is to compensate for not having extra money while “at war during collect income”, for example:
The US is attacked turn 2, gets 30 IPCs immediately so they have that to spend when it becomes their turn. If this were NOT the case, then the US would only have their initial 30 IPCs to spend during “war time” which wouldn’t make sense.
I will offer up a rationale for the tac/tank combo. Think about Panzers and Stukas for a moment. The Panzers are used to pin down the enemy in known locations. Then Stukas are sent in to finish off enemy defensive positions before the Panzers blitz the line. If it weren’t for the Panzers pinning the enemy units, they would be harder for the Stukas to hit.