Eevee: For outsiders, it's really easy to say 'Just leave it and go.'
If I remember correctly, she had that Eevee since she was...a child or something. Young, I can confirm.
Now, would anyone really abandon something that they share a close bond with? Think of it like a pet.
Say you were in Mitsumi's situation, except it was like...a cat or something. This random evil dude kidnaps it and demands you to join the evil side or your precious pet dies. At least, I think he said it was going to die. I forgot =DDD
Save your pet(which you love to pieces) or battle your friend(in which you met only 2 years max. ago). Now who would you choose?
Mitsumi was probably so attached to her Eevee, she was willing to do anything to save it. Which is why she decided to join Team Galactic.
No worrys though, Hareta will save the day! ^^;;
Palkia: Palkia should get a bit of screentime. Maybe Shigekatsu Ihara can't draw Palkia xDDD
Sorry if this post was confusing. xP