Posts from About changes and/or combining Hero Specializations. Auto-translated

That's correct.
To create a special ability for Liches, you will either need to modify the special ability of an existing Necropolis hero, such as Orsen.
Or, you can "borrow" from another Faction, for example, "steal" the Orc special ability from the Orcs and systematically swap it with the Lich special ability.
You can also create a "non-standard" special ability, such as the Witch special ability for the League or the Wyvern Tamer. You can "steal" the Centaur special ability from the Orcs (its difference from the standard one is that they also receive an attack bonus at the beginning of the battle, proportional to the hero's level) or the Tier 1 Gnome special ability (which, in addition to stats, also buffs HP) 🤔

The main thing to remember is two simple rules.
1) If you simply modify an existing special ability to match another Tier of your Faction, there will be no problems.
2) If you take a "foreign" special ability for Creatures, for example, from another Faction, don't forget to give them another special ability in return, or remove their Hero from access.

Are these definitely verified conclusions?

Because when I was testing and adding a third tier of creatures, I thought that the creature specialty was hardcoded to specific creature IDs. 
But when I started adding them, it turned out that the specialty works on new creatures as well, if those creatures belong to the same faction and the same Tier.
For example, Razzak buffs golden golems with his bonuses — these are new IDs, but they belong to the Academy and are also placed on Tier 3.

Thank you for the clarification! If this is indeed the case, then even though this option doesn't provide complete freedom of action (you still have to replace something with something else), it nevertheless creates a certain space for experimentation.