Actually, I think I can solve both problems with 1 solution. Making the other hero playable. The second hero is weaker and relies on mana, which would definitely create a hardmode situation if he is by himself.
Thought as much, still I'd like to play as the other hero, and I see no problem controlling them both actually!
Did you defeat the sidebosses, what did you think about them? (There are like 4 of them + sidequest)
Yes I did, very creative as I stated. Especially the treant one, having to avoid that massive damage while trying to figure out how to quickly destroy it was fun. Quick thinking won that for me otherwise I'd had to restart from a checkpoint.
About the sidequest you surely mean the necklace and the necromancer?, it was a bit hard since I started from the south-west and moved towards the north after I lost the village there. But I completed the quest, unfortunately by that time I had already taken down the necropolis, didn't get to enjoy the benefits of killing that necromancer.
How did you feel about the secret items (if you found them) and how was the north village mini boss(since you said you couldn't defend it).
I believe I have found all the secret items, I couldn't use the scepter of course since "my brother wasn't there" *sob*
I didn't know the shop items would be laying around though. I had bought what I needed when I found them anyway, Lucky ring and hero's amulet seemed good starters until the secret items are found.
The golem you must presume?. As I said earlier, after obtaining the sword and quickly rushing to the village I had no problem dealing with that, since the footmen were there attacking it. All I had to do was mass spell him and finish with a nice holy light to heal myself a bit.
The problem was with the damn zombies, they kept resurrecting. As soon as I thought I had finished them, another wave came of 4-6 zombies and 2 ghouls, I got bored and thought it's neverending so I abandoned the village proceeded south to the bridge, explored the west and north later. I never understood why the gate is closed there though.
Oh, and "Red Baron" was a nice touch after the village had fallen
About the graveyards, the undead will try to build a graveyard in certain key locations, while humans will try to build towers in those locations. The AIs already fight for area control and will decide what to do(also taking into account gold)
I understood as much, perhaps I outdid myself because the only checkpoints they had was a graveyard near their base. The painful part was getting rid of the occasional zombies with poison attack, they kept slowing me down bah.
As I said, the only thing missing is that other hero/unit.

Edit:
I really enjoyed the gameplay, are you planning anything similar for the future?. Perhaps something lore-related. Not linear, simply chronological. And by similar I mean something blizzard like, short and creative at the same time.