I can give you an answer on 3, straight from the FAQ section on the developer's discord:
"How does the reserve system work?
You can field X gold value of units at any given time. At the start of combat, you can bring X, and when you've lost half of X, you can bring in half of X as reinforcements. X is 10k gold value as base. On top of this, X is increased by 10% of the gold value of YOUR army and 10% of the gold value of the ENEMY army. So if you both have 15k gold value armies, the reserve limit is 13k (10k + 10% of 30k). This number is SYMMETRICAL for both armies. Finally, Tactics and Oligarchy can increase or decrease this number ASSYMETRICALLY."
That might explain why you can't field more units, even when facing a "near impossible" army. You can counter that as stated with "Tactics". And, of course, your hero's stats determine the stats of your troops if I am not mistaken. So if a Lvl 5 hero faces a Lvl 15 hero with gear, that might be the reason why it is still "near impossible".
Edit: Join the discord, the developer is a cool guy. He tends to respond pretty quickly to your feedback and/or bug reports there.