It is interesting just how unimportant 1 Corinthians 15.5-11a is for the rest of the chapter:
5 ...and that he appeared to Cephas, then to the twelve. 6 Then he appeared to over five hundred brothers at once, most of whom remain until now, but some have also fallen asleep. 7 Then he appeared to James, then to all the apostles, 8 and last of all, as to a miscarriage, he appeared to me also. 9 For I am the least of the apostles, who is not worthy to be called an apostle, because I persecuted the assembly of God. 10 But by the grace of God I am what I am. His grace which was given to me was not futile, but I worked more than all of them; yet not I, but the grace of God which was with me. 11a Whether then it is I or they....
Verses 3-4 at least mention the death and resurrection of Christ, and the rest of the chapter is all about the resurrection from the dead. But here six and a half solid verses intrude which find no reflection at all that I can tell throughout the rest of the chapter. This is one example of the kind of reasoning that Paul employs in the chapter:
12 Now if Christ is preached, that he has been raised from the dead, how do some among you say that there is no resurrection of the dead?
We have preaching
(tying back to verses 1 and 11b), and we have a raising
from the dead
(tying back to verses 3-4), but we find nothing corresponding to an argument from the appearances
, to wit, "Now if all of these worthy men are witnesses of the resurrection of our Lord, how do some among you say that there is no resurrection of the dead?" (This observation overlaps with some of what spin was arguing on the Reddit page linked to in the OP.)
So why are the appearances here in the first place? If they are apologetic in nature, listed in order to prove the truth that Christ rose from the dead, why does the argument of the rest of the chapter proceed as if they had not been listed at all? This seems like naming a premise in a geometric proof that the rest of the proof then fails to reference.
It has also been suggested by some that the list of appearances would serve as credentials for the various leaders in early Christianity (Cephas, James, the Twelve, the apostles). But the argument in this chapter has nothing to do with apostolic credentials.
Furthermore, Paul takes up three and a half of these verses discussing what the chronology of his own appearance event means; once again, none of this resurfaces in the rest of the chapter (nor, indeed, in the rest of the epistle). It is a digression inside a digression.
But hey, authors sometimes digress, right? Perhaps these verses are
a good summary of Paul's overall preaching, and he is including all of it, even the bits that do not apply to the current topic, and even when they distract him into adding stuff about his own experience that can scarcely be part of the original gospel message. I myself would argue something like that for two particular details on my own preferred (Marcionite) reconstruction: the burial and the third day, neither of which figures into the rest of the chapter. But of course there is a huge difference between carrying over two tiny details like that and carrying over what amounts to the vast majority of the passage. And it is at least interesting that, if the entire passage is indeed a good summary of Paul's gospel, the appearances to the other people never seem to come up for discussion in any of his extant epistles (whereas the death, the burial, and the resurrection do).
Also, while it is difficult to come up with a good reason for Paul to have digressed so far so quickly from his main objective in the chapter, it is quite easy to come up with good reasons for later churchmen to have added all of this material. The appearances ensure, against Marcion, that Paul is not a maverick in the church. The "last of all" speech ensures, against Marcion, that Paul himself felt subject to the leadership, exactly as Acts would have it. (And the "according to the scriptures" lines ensure, against Marcion, that Paul is preaching in accordance with the Jewish scriptures and not in direct opposition to the Demiurge whom they celebrate.)
There is no smoking gun here, I freely admit. But they are considerations to take into account.