I have little question about this mission. Have someone tried defeat attack aerodynes that arrive after 20 minutes count at end of mission instead evacuating before time limit ends? More experience for destroying them could be nice but I'm wondering if it's possible... I'm playing on normal difficulty?

I think it may be possible. I believe in that mission you can load 4 light tank squads (equipped with deployable SAM kits) into the dropships and lay down them in the (now fuming) Dawnie's base. You can have a maximum of 12 SAMs (3 for each squad) that way and the enemy reinforcement consists of about two or maybe three squads of attack aerodynes. I won't say that's an easy win but you can take chances.

Never tried it myself, though.