why couldn't Cephas and James have also only experienced a celestial Jesus?
Doesn't James do exactly that in the Gospel according to the Hebrews and various Nag Hammadi texts written in his name. The Gospel according to the Hebrews:
Also the Gospel according to the Hebrews, lately translated by me into Greek and Latin speech, which Origen often uses, tells, after the resurrection of the Saviour: 'Now the Lord, when he had given the linen cloth unto the servant of the priest, went unto James and appeared to him (for James had sworn that he would not eat bread from that hour wherein he had drunk the Lord's cup until he should see him risen again from among them that sleep)', and again after a little, 'Bring ye, saith the Lord, a table and bread', and immediately it is added, 'He took bread and blessed and brake and gave it unto James the Just and said unto him: My brother, eat thy bread, for the Son of Man is risen from among them that sleep'.
I don't understand the 'normal' (= white people) reading of this passage. James is sitting in the tomb with the servant of the high priest or on his own having a meal? Really?
There are only three possibilities here:
1. the story was made up by someone who knew nothing about Judaism
2. the meal did not happen at the tomb it was 'an appearance' somewhere else
3. the author and his community did not really believe that Jesus had died (or lived for that matter) so the tomb wasn't ritually impure.
Since dead bodies and graves are a source of ritual impurity, the Jewish cemetery is usually situated beyond town limits. The cemetery is also invested with a certain sanctity, and any activity therein that might tend to show disregard for the dead, such as eating or drinking, is forbidden. Out of respect for the dead, individuals are not walk on graves, except where it is impossible to get from one grave to another.
So what were Christians doing gathering in graves from the very beginning? There must have been the idea that Jesus was a supernatural being and so 'didn't really die' or the person who died in the tomb was made supernatural so the normal rules of impurity didn't apply. The fact that Christians CONTINUED to gather in graves and eat meals surely horrified Jews (especially if we imagine a large conversion from Judaism or among Jewish proselytes). Nevertheless a strong case can be made that some sort of 'nascent mythicism' is at work here - i.e. Jesus didn't really die because he never 'lived' (in the mortal sense) - if the place of the 'appearance' happened somewhere other than the tomb.
And if so, James had an appearance happen to him like that of Paul ...