Do we get the source files?

They did implement a workaround for water elementals in the form of a 70% damage reduction from spells. Yes, it's not the best solution, but drastically changing the game's mechanics by limiting the attack range doesn't seem quite right. I think the 4th point, if implemented correctly, should fix the situation. And the 7th point as well. Is it worth focusing on this problem from all angles? You could overdo it and make things even worse than they are now.

Well, maybe, I can't say for sure. But here's what needs to be noted:
1. Some archers are also a problem, not just casters. No one wants to run into blue elf archers.
2. The unit shouldn't be an overpowered neutral enemy, but it should still be useful in the hero's army, including in the final battle against another player. To achieve this, a weak unit needs high damage. You can make all archer and caster units balanced (average damage and average survivability) or focus on survivability, but then we lose an entire potential class of creatures. This won't diversify the tactics.
3. There will be more point in maneuvering, which is a plus in itself.
In any case, such global changes imply a complete overhaul of the game balance, of course. But I believe it can be balanced in such a way that the tactical possibilities are actually improved, at least some examples of this exist.

No range penalty perk

Oh, maybe. But I should say that in HoMMV it's not so big problem because there is not so many shooters with no range penalty. Have you tryed HoMMVII? ) If yes, you know the example of another situation here.

If you use the proposed nerf from a couple of posts ago, archers will become irrelevant in comparison to the other units. Have in mind that tower has 3 archer units which will greatly alter their strength.

I don't think so because the ability to move in the same turn with the attack compensates it! Of course, it will be harder to use shooters, but they will get ability to easily occupy advantageous positions on the combat field. Also it can be balanced by stats (damages, vitality and so on, as well as a distance of the attack!).