King's Bounty: Armored Princess

Yeah, but there are a few problems:
a) I'm still a weak player, and the normal difficulty level seems impossible for me;

The strength of the game, just like physical strength, only grows if you set yourself an increasingly challenging task each time.
If you do one push-up a day and don't try to do a second, you'll never do more than one. If you don't challenge your brain with an interesting task, it won't try to generalize, analyze, or draw conclusions. What's the point of playing then? You can just watch a cartoon. Turn on auto-battle, and the computer will do everything for you. And it will congratulate you on your victory. And it will even eat for you. :)

And finally: what happens if you upgrade a unit with a squire and there isn't enough leadership for it?

You can always move the extra soldiers to your pocket, castle, or destroy them. The operation is performed by pressing the Shift key and the left mouse button.