Thanks for the Feedback!
1) Since the first completed level gives 50% bonus now, then half of the first level should give half of it. We deliberately lowered the defense bonus based on alot of feedback from veterans so we currently don't intend to increase it again.
2) The point of Inf in a stack is that it protects your expensive powerful units. Inf is basically cannon fodder. As someone who builds a tank you probably want your tank to survive and not your Inf because Inf you get basically for free. But in the previous version Infs blocked a bit too much damage, that's why we made to block less damage from other units. Note that the total amount of damage for the whole army will remain the same, this is only about the share of damage each unit in the stack gets when it is a mixed stack. The new values were shown in the changelog. Inf was reduced from 1.0 to 0.5 for example, meaning half of the damage that Inf previously received will now go to other units in the stack.
Your suggestion with configurable battle rows is nice and we thought about it many times in the past. But this is essentially a new combat system which is a big task. Currently it is not in the scope, so won't be coming any time soon, sorry.
3) The balloon change is currently being discussed internally. It may be reverted in the next release.
Regarding the reported bugs:
1) Yes that is known and was like that since forever, that is sadly due to how the engine works. Would be expensive for server performance to do it differently, but maybe it will be fixed at some point (not soon).
2) noted.
3) I doubt that this universally the case, it might happen in specific situations due to the time interval in which the server updates unit states. Unfortunately it is too expensive for performance to update units all the time, so there are intervals, if you are lucky the next update is shortly after the enemy unit entered the attack range.
4) Its a point of discussion, maybe it will be changed at some point but sadly it also enables exploits. In the engine airplane attacks are similar to ranged attacks, so for example you could block gold usage just by giving an attack command to airplanes even if you have no intention of actually attacking. So stuff like that needs to be fixed first.
5) noted.
6) Expansion and Distance are not the same thing. Distance only affects far away provinces, Expansion affects all provinces. Similar to how previously there was distance penalty plus "at war" penalty. The at war penalty was also for all provinces, but it got replaced by the expansion penalty.
Currently the penalties are beign reviewed, there is a chance that they will be revised in a future update.