Excellent educational material! Thank you!
1. Why did Ignissa enter the portal? And how did Ignissa return from the portal? She had no movement points left. And even if she did, wouldn't she need
to step aside and accordingly take a fight with one Pixie? What trick is that? And why did she return at all?

A space is our clue.

2. How can you see on the map that whirlpool that Sylvia fell into? It's not visible. And explain these words: "By using transparent ponds, we summon a
boat with Sylvia"?

The spell "Summon Boat" from the Water magic school. The terrain where Sylvia summoned the boat is called "clear ponds". On this terrain, all Water magic spells are cast at "expert" level. There are no free boats on the map, so without expert Water magic we cannot summon a boat. The terrain helps us with this.

3. Why did Kilgor swap a Behemoth for 1 Gorgon? How is a Gorgon better in combat than a Behemoth? Explain these words too: "with the help of the gorgon we headbutt the rusted ones (struggled)"?

The Gorgon has the "Death Stare" ability. It was used to kill the Rust Dragons.

4. How did Sylvia transfer all items to Zeron? She had no movement points after landing, and Zeron, as I understood, didn't move.

Sylvia landed in front of the Gold Golems, fled, and was recruited from the tavern.

5. Corbac is standing in the Sulfur Lair, the flag color is gray, in the next save it's green, but the dragons aren't bought. The question is, how can you enter if you've already entered? Step away, then enter again? Also, the unicorn dwelling was taken. There must be some trick?

Space.

6. When breaking the Pandora's Box, what was the trick based on, what's the point? I mean, how do you break it?:)

In the first round, I removed the retaliation of the Harpies with the Centaur (kicked with everyone except the Troglodytes).
Unicorns blind, better at the start of the second round. The boss moves to the eyes, Troglodytes approach the Harpies.
In the third round, Unicorns go to the eyes, everyone defends.
In the fourth, everyone waits, Troglodytes attack (they survived retaliation, I think), after the Harpies' turn (they finish off the Troglodytes), attack them with Centaurs and Unicorns. Unicorns blind in two hits (in the 4th or 5th rounds).
Then the same as at the start. Remove retaliation with the remaining single Centaur.
http://zalil.ru/33741880

Explain what this "breaking" means and why?

We attack and flee, in order to then recruit from a tavern in a different location on the map. That's the trick.