Does Parthia (Eff+) does it stack with the Defect Magic/Missle?

It works fine for my Jeorge with a similar setup. As you put, it's basically just like Sigurds and a few other heroes with these skills on their weapons. It doesn't seem to me that it is not working from the results I've had. It can still be affected by certain characteristics like colorless weapon damage and such, but it should still reduce what you're ultimately being hit with during both the first and second attacks (and onward) to him, if they fit the criteria.

I am guessing that you retaliated between the hits? Then yeah it will not proc any skills on your opponent second atk. Parthia only work on the very first atk of the turn(like Divine Tyrfling) and the Deflect seals only work against the second attack in succession, like a brave effect or if you initiate and get double afterward or if you are melee and do not retaliate. This is why DC Sigurd and Seliph are wack, and seal skill, not very good. At least any archer can have a brave bow, so Defleck missile is not as trash as deflect magic

"Consecutive attacks" means they aren't interrupted by your own attack, which is what I assume happened. Hence, the seal didn't come into effect.