How do legendary respawns work in BW2?

Question

I recently purchased B2 and read on serebii, that some legendaries -and Volcarona- appear as lv. 35 the first time, but as lv.65 the second time, after beating E4.
My questions are:
Are their natures set upon the first encounter? Can I defeat Volcarona at lv. 35, then come back after E4 and only then start trying for the right nature at lv. 65?
Do I have to battle them the first time, or do they simply become lv. 65 after E4 is beaten? (and therefore, can ignore them until I finished the game)

Comments

Experience: They are at their lower levels by the time of the first encounter. I won´t attempt second encounter, as they may have the wrong nature forever (as beating E4 is followed by saving the game), which I would dearly dislike. Everyone, thanks for trying ;) Keep on the good work.
Also, I´m keeping this question here, should anyone look for an answer.

---

after beating elite 4 again, they will respawn

Answer 1

Very well, after trying it out:

• They need to be beaten the first time,
• and it is possible to safely beat them, come after E4 and only then start hunting for the right nature.