Huh? I just mentioned he was an observant jew. Most (even Catholic scholars) would agree with me.
And while the core of the Passover has not changed, there have been some modifications to the liturgy. I'm not aware of anything that would impact this "fourth cup" argument.
Maybe in reform Judaism. I doubt orthodox.
I think you're missing the point. Yes, I agree He was an observant Jew. The Jews at the time didn't think so (witness the trial & crucifiction for blasphemy, working on the Sabbath, etc). Jews today don't consider him particularly "observant" either since observant Jews don't claim to be the Messiah.
He either was what he claimed to be or a liar. Only if you believe He was who He claimed to be can you say you think He was an "observant Jew" (and in that case you are forced to re-re-re-re-re-convert.