When will exactly the snapshot of IGNIS be
The exact date is not known but should be between October (ICO end) and the end of the year. I guess November/December.
[...] and what do you think how will Ardor develop further? Sidechains are a mega trend when more and more people and companies are using crypto currencies in my opinion. Ardor will probably offer the first integrated solution for sidechains. I think it will have a bright future.
Yep, that is also what I think will be the main feature. However, the value of tokens in "child chains" in Ardor won't be pegged to the main asset (ARDR) or to IGNIS, so the use cases of child chains are a bit different as that of Blockstream-style side chains.
Will the main developer Jean-Luc also switch to Ardor?
Yes, Jean-Luc is already developing Ardor.
Does it make sense to exchange the Nxt after the snapshot in Ardor?
In my opinion it makes more sense to exchange Nxt NOW into ARDR (preferrably on the NXT Asset exchange), because its price should fall towards the end of the snapshot period and even more so after the snapshot. Or alternatively participate in the Ignis "ICO" on the NXT blockchain. The ICO is however massively undersupplied (until now) so it's possible you won't get the coins you want in the next (still cheap) rounds.