Ad aspera

How quickly does the transformation take place? For example, there's a guy in armor and a helmet, he sees the guards and knows there's a reward for his head, can he quickly change his face? Or does it require a lengthy ritual and cannot be used on the fly?

It doesn't require a ritual, but it will take about a couple of minutes. In terms of gameplay, this means that it is impossible to react instantly, but if the player understands what he is doing in advance, nothing will prevent him.
Conditionally, "while passing through the door, the Warlock changed his appearance and entered the room as a completely different person" is not possible. "The Warlock, a skinny and bald little man, turned into a deserted alley, and emerged as a handsome young man with a striking red head of hair" is possible.

Can you use any gear - weapons, armor, shields, ranged weapons - even without a specialization? Does the specialization in this system only determine the possibility of receiving additional bonuses?

That's right, you just need the strength parameter. The only thing that imposes penalties on its own is heavy armor.

Does ranged weapons require a skill, but there is no specialization in the branch, which means that you either just use it with the base stat, or not?

Yes. Ranged weapons receive a strength bonus, can be used while moving, and have additional effects tied to the item.

How is parrying calculated with a one-handed weapon in conjunction with a shield block?

The priority is parry-block-dodge.

Is the "Mastery" from the armor branch considered a dodge for abilities that allow counterattacks?

No. Except for the "tank," of course.

What is the duration of the fire ring? Does the opponent take damage automatically, without the possibility of a defensive roll? Is the ring static or does it move with the caster? If there are opponents within a radius of 5 cells around the caster, do they automatically take damage? Do characters who have completed their turn in the wall of fire take damage at the beginning of their next turn?

3, I'll correct that. Yes. Static. They will. No.

Something else I remembered: for a counterattack, the opponent must be within striking distance. That is, if you take the second rank in the polearm skill, you become immune to counterattacks from one-handed weapons.
If the attack roll for an effect is three times greater than the defense roll, the success is automatic. This is done so that a "yo-ba" mage with 40 willpower does not try to cast the only mind control spell of the game, fails, and gets hit by a random guard.

Added 38 minutes ago
Oh, no, I lied a little. You can't use a shield, it's tied to the first rank.
Maybe I'll give one more test build. Now I'll finish the religions and see the overall picture.
I still think I'll leave the magic accuracy check against the sum of checks so that both the melee fighter and the archer have a chance when there is a superiority in stats. The only thing is to tie magic accuracy to intelligence with +5 per point, maybe.