What happens when you attack with Melee Robert and use Direct Assault ?

Staton said:

Robert Baratheon gets +2 STR. If you win, I assume first player chooses which replacement effect is enforced.

Ahhhh I didn't see Robert as a replacement effect. Touche.

Yup. The first sentence (everyone can declare a defender) and the second ("instead" of normal claim) are completely separate effects. (Just like the "+2" and "instead of claim" on Direct Assault, I might add.)

So Bomb is correct - you have 2 "conflicting" claim replacement effects. The First Player decides the order in which they are applied, with the last one applied being the way that claim resolves.