Question - were there any Towers in the enemy Fortress, and if so, how many?

By day 8, no computer will have time to build even a Stronghold, meaning there are no Towers at all.

In my opinion, 43 Gremlins survived in this battle.

Apparently, I got a little confused and provided not quite the correct record, as I played this battle almost identically several times. In the original Walkthrough, 42 Gremlins survived. It turns out that the one you watched was even slightly better (by 1 Gremlin). But these are all minor details, as the final battle was played without any problems.

And here it's unclear - where did Narhiz get 63 Gremlins and 8 Gargoyles in his army in this battle? After the battle with Gilraen, there were only 43 Gremlins left? I didn't understand from the description where he got reinforcements.

Nur ran into the Castle and, leaving the Gremlins, left, and then a mage with his own troops was bought in the tavern (day 8) and went to fight.

So, how many hit points did your tent heal? I got the impression that my tent worked worse - what's the secret?

The tent heals depending on the skill in Battle Machines - 10/20/50/100 HP, except for the hero Kaspar, who has 5 additional healing units per level.

And can the tent revive a killed unit during healing?

Yes, it can, this is its harsh difference from the third game. In the fifth game, Battle Machines are great.

I already know that Hydras regenerate on their own.

Only upgraded Hydras regenerate. In general, in the fifth game, creatures have even more abilities and special properties than in the fourth, not to mention the third, and they are very often decisive.
Iron, please read the manual, and recommend it to Luchik as well. Then I am sure that your duel will be even more interesting and you will compete for the top places. Because a lot more has changed from the fourth game to the fifth than it might seem at first glance.