I believe St. Paul is considered an apostle.
I think you are misunderstanding the protestants, though. The point they are making, and I think it valid, is that both Paul and Matthew are eye witnesses. Paul did not know about the life of Christ because the other apostles told him about it, he knew it because it was revealed to him by God, just as it was the others.
Thus, I think you are making a false dichotomy in saying that they should not focus on Paul, but on Matthew. It seems you are saying that the two may be contradictory. This is not the case, you should focus on Paul AND Matthew. Both are correct, and both apostles convey the Catholic belief in the real presence of Christ in the Blessed Sacrament.
You may be trying to say that one is more clear than the other, which I think would be accurate (even Peter mentions that some of Paul's writings are not easily understood). Plus the Gospel account does provide Christ's own words, which I do agree makes it particularly important. Both are completely 100% correct, though (as I'm sure you agree).
IMVHO John 6:60 sums up this entire discussion very nicely.