Since he had yet to be crucified, I'm not sure how anyone could argue that his presence at the Last Supper was anything but a real physical presence. I would have to say that in this case, anyone who argues differently is simply wrong. Who claims otherwise?
With regard to Paul...I believe his letters to the new and growing Church are absolutely vital to understanding our relationship with Christ. The book of Romans was like the Rosetta Stone for me to understanding the rest of the Bible. I personally don't know anyone who believes that once you are saved you are always saved. Certainly Christ or Paul never said such a thing. In Jesus' Words, "He who has believed and has been baptized shall be saved; but he who has disbelieved shall be condemned." If I once believed that Christ died for my salvation, but then I decided that was not actually true, I have rejected Christ, and am no longer saved. Who believes differently?
Paul clearly does not believe justification and salvation are the same thing. Take a look at the book of Romans. He starts by explaining justification. He then discusses salvation. Never does he say we are saved by faith alone. Anyone who would argue that, is ignoring the words of Christ and Paul. But let me add here, Paul also says very specifically that we are NOT saved through acts. The penalty for sin is death, and we are all sinners. In God's eyes, sin is sin. We have all fallen short, and there are no acts that could change the fact that we are sinners. It is only because Christ's sacrifice has paid the penalty for our sin that we are justified, and even with that, if we reject Christ as our savior we are condemned.