Lets say I have ben grim on the field as well as the FF4 pog allisha masters and a healer can I choose to replace bens def with alishas for the heal roll? If yes would it be the same if someone attacked me on my opponents turn or could i use bens nat def?

Thanks in advance

im pretty sure its optional when you can share defense values but im not entirely sure

but i really cant see why not

You could use Alicia's 10 DV on Ben Grimm for the Support roll. The Feat card just specifies that you replace the DV of one character with an unmodified DV of another character, it doesn't say anything about having to use the highest one.

Also, you can switch the DVs at anytime. The Feat card does not specify 'once per turn' or any other limitation on the number of times you could use different DV's. So Ben could use Alicia's DV for the Support roll, then on your opponent's turn, use his own DV, or Human Torch's, or Sue Storm's, or any other alternate FF member.