But that would be our interpretation of what St. Peter understood not what St. Peter understood, had problem understanding or failed to understand.
It would be as if we are able to determine for the Holy Spirit what He made known to the Apostles (as many seem to want to believe that they are able to do today).
What are these things that are hard to understand?
Why would the Holy Spirit Inspire a Writing that only St. Paul could be able to understand?
How would the Church Teach in the absence of St. Paul, would the Twelve not have to answer, ‘hey, we don’t fully get it either, let’s write to Brother Paul to see if he can come and explain it to us or in the least write an unabridged version so that we can read it to you?’
We have problems because we fail to yield to the Holy Spirit–like the Ethiopian and Philip.
You’ve answer your own statement–the things that are hard to understand can be understood through the submission to the Holy Spirit’s Guidance; it is the reason why even when Apostolic Teaching seem conflicting we can know that it is not.
People who opt to go it their own way do get derailed and do end up distorting God’s Word.