Can I select which ability to use on my second card after using the first card?
Yes, I believe you can believe you can choose to use Move 2 instead of the attack on the bottom half.

Like you I've struggled to find anything explicit to state this but I'm going back it up by things the rules don't say.

Firstly players reveal cards for initiative.  There is nothing here about stating what card abilities you will you.  I've have experience of planning to use two actions but by the time my turn came around it was more beneficial to use the opposition actions of what I originally planned.

Then on your turn, you will then choose to play the top or bottom half of one card in either order.   Again the rules are clear here this can be the printed ability or attack 2 / move 2.  You will then play the opposite side of the other.  Again this could be attack 2 / move 2.

What is important here is the rules is the following,


  Either card can be played first for its top or bottom action.


and 


  As soon as the action of a card is completed, it is immediately placed in the appropriate area.


also 


  When playing a card’s action,


So you completely resolve one card, and then the other.

The rules DO NOT SAY you must declare what actions you are doing on both cards at the start of your turn.  They infer you only decide what action to take (either the printed ability or the attack 2 / move 2) 'when playing' that card.

So say you choose the top half of one card "when playing" that cards action you can chose if you use printed ability or Attack 2.  You then move that card to the appropriate discard area.

Then you activate the bottom half or the 2nd card so 'When playing' that card you can again choose if you printed ability or Move 2. 

How you ask is certainly how I've played it.  If the top half attack kills a monster then using the bottom half for move 2 instead of the originally planned attack is probably a sensible thing to do.