@captain0
Thanks for engaging.
The upkeep works well and it is a shame to have a 2 fort limit but I guess that is because of AI.
This could be increased but it might feel too much like WW1.
When new players come accross Bunkers, they struggled to take them, so I never thought to increase them in number. With the current stacking of other units and of 2 bunkers, it feels about right to give the Atlantic wall feel/vibe and Russian attempt at defence in depth and 1 Bunker in the Pacific gives the dug in Jap feel.
.
static fort upkeep be halved?
Upkeep is in whole numbers only, 0 to say 9, so no .5 allowed.
To get the equivilent of .5 you would have to double all PU purchase costs including upkeep to 2, except Bunkers they would be 1. This might mess with the current balance and it might put off human players that are so used to A&A PU costs. The Bunker upkeep cost of 1 also deters the human player from spamming them.
.
I set Russia and Japan as allied for a historic game and change that when war in Europe finishes,
Yes the current NAP is weak, why would Russia declare war on Japan when it is losing to Germany. A reworking is on my TODO list 
.
I believe historically the Finns were averse to advancing further than the Vyborg territory that they gave up in the Winter war..
The Finnish commander Mannerheim had Soviet roots and Finnish people were not hugely anti Russian in general.
We wanted to keep the minor powers to a minimum, although realistic they slow the turn speed down, so a compromise, no Finns, just more Germans.
I hope others chime in with thoughts to your questions.