Yes, more than one round seems a bit extreme, to me.
Here’s my rationale:
Capitols are probably built up with bunkers, barricades, trenches, reinforced buildings, strong infrastructure for communication, closed communication lines, closed supply lines, etc. However, on the first wave of the attack your attacker is hitting you with artillery barrages, bombers, fighters and rolling through with tanks. That means you’ll be losing a lot of your defensive fortifications, and thus, be reduced to defending like any other territory.
Secondly, from a mechanics stand point, having all infantry defend at 3 or less after the first round would seriously stop capitol assaults. It’s too powerful. Even Atlantic Wall and Fortress Europe (and Dug-In Defenders) are limited to a first round defensive boost, probably for the same reason.
Lastly, Dug-In defenders are already a rule. We’re just expanding that to include capitols, which makes sense because of point 1.
Further thoughts on “snowy” terrain. I don’t think I’d use that at all. Instead, assuming the point was to lend Russia some aid, why not just state that only Russian tanks may blitz through red territories? (as defined as painted red on the board, not captured by Russia.)
This would stop England and America from blitzing as well as Germany and Japan, but it would hamper Japan more (not critically) then any other nation.