Does this work?

I'm 99% sure it doesn't work, just wanted to confirm.

I confirm

I agree it shouldn't work. There seems to be a distinction between "does not count its defense" and actually having 0 defense from cards that reduce defense.

Doesn't feel like it would to me... The enemy doesn't have 0 defense. Ignoring isn't the same as reducing or removing.

A sillier one: If I'm attacking with Mirkwood Runner and mid-attack play a Quick Strike to attack the same enemy with another character, does the enemy count its [D] for this new attack?

No, because Mirkwood Runners ability only works for the attack he is currently performing. Otherwise it would be worded "until the end of the phase".

2 x Rivendell Blade attached to Tactics Aragorn equiped with Elf-friend will work in most cases with Straight shot.