According to the wiki, event girls don't have HP drain after promotion (that's a shame). It's a hard 5.4x damage to 1 enemy and her black point. It don't pair well with the +15% ATK depent on enemy number.
About Pinkladies, I didn't notice she will turn into a good Kerria-like girl after promotion. But she remains slightly under Kerria with less HP, less ATK and just a little better DEF after buff. But she don't have the low speed issue.

About the reduce enemy acts ability, it should also prevent single attack enemy to act right? Ability says: "Each turn, has a 25% chance to reduce the number of times enemies can act by one.". If you reduce one act by one, the enemy should not attack at all. If it's the case, it's a bit broken and it would be helpful on many missions, hardest or not.