Bekah the Bard has the power "For your check that has or is against a card that has the Finesse trait, you gain the skill Melee: Dexterity +1".
If her Dexterity skill has a +1 modifier does that get added as well?
Yes.
Nice! With the Troublemaker role which adds a (+3) option and a maxed out Dexterity (also +3), she becomes a much better fighter.
I'm assuming Blessings of Erastil (2 dice for combat Dexterity checks) works on this as well?
If you're using your Melee skill to fight - yes, Erastil works. If you're using it to acquire a Finesse weapon - you want a Calistria.