QIf I select a Galmieux, Omen of Disdain with 2 or less defense on my field for this spell, can I still "deal 2 more damage to each enemy follower on the field"?

AYes. Even if your Galmieux, Omen of Disdain drops to 0 or less defense, it doesn't get destroyed until you finish resolving this spell.

QIf I can't select a follower on my field, can I still play this spell?

ANo, you can't.

QIf I have Galmieux, Omen of Disdain on my field and my opponent has Elf Girl Liza on theirs when I play this spell, how much damage do I do to enemy followers on the field?

AYou deal 2 damage to each enemy follower on the field twice, with each instance of damage reduced by 1 with Elf Girl Liza's ability. So, in total, you do 2 damage.