And those who opposed identifying this Cephas with St. Peter make the same argument, that his abrupt reversal of his practices doesn't really make sense.

I think it's perfectly in line with St Peter's personality (i.e. sanguine) to make rash, impulsive and contradictory decisions, especially out of human respect.  Holy Thursday is a great example of how St Peter pledged his allegiance to the Lord and then a few hours later, denied Him.  Then we have the the story in Scripture where Our Lord appeared to St Peter, as he was walking away from the persecutions in Jerusalem.  

The sanguine personality (as its worst) craves attention, needs approval and is often superficial and unstable.  All of the stories where St Peter made mistakes are explained by his personality defects.

No, St. Peter was NOT one to act out of human respect.  Was he sometimes impulsive?  Perhaps, but much less so after Our Lord's Resurrection.

 St Peter's entire denial of Christ was based on human respect.  

He most certainly would not have compromised doctrine out of human respect.

Agree.   

It's possible that he did so in order not to "scandalize" the Judaizers, which was a mistake on his part.

This is a form of human respect.  To change actions (or the appearance of beliefs) based on the opinions/needs of others.

But the R&R abuse of this passage to pretend that St. Peter had taught error to the Church (when he clearly had declared the exact opposite at the Council of Jerusalem) is completely unfounded.  St. Paul himself said it was OK (and even required by charity) to refrain from taking certain actions to avoid scandalizing those who were weak in the faith, and perahps that's what he was doing here ... again, if this were St. Peter.  I don't buy that he acted out of human respect, and he most certainly did not compromise his teaching on account of his actions.  As I mentioned, the Church Fathers who had the theory that this was play-acting for public consumption had the same motivation, the inability to explain such an about-face.

I don't think he taught error, nor did he compromise any teachings, but its apparent that his actions were wrong, and it was out of motivation for others, even if this motivation were well intentioned.

St Peter's entire denial of Christ was based on human respect. 

I would argue more abject fear than human respect.  He was terrified that if he were outed as an Apostle that he too would end up crucified.

Nevertheless, yes, the Apostles failed during that time.  After 3 years of being with Our Lord and witnessing His power and His miracles, raising people from the dead, and then telling them that He would rise again after His death, still none of them believed Him.  Our Lady and Our Lady alone kept the faith during that dark time.  That is why she wasn't among the women going to anoint His Body on Easter morning.

Even though the Apostles were highly flawed individuals before Our Lord's Resurrection and even before Pentecost, Church Fathers and theologians hold that they were all confirmed in grace at least after Pentecost.  And the cowardly men hiding behind closed doors were transformed into heroes of the Faith and of the Church, intrepid in every way.  Despite being around Our Lord for 3 years, James and John were still jockeying for position in the Kingdom of God.  They all fell asleep in the garden, and all but St. John fled during Our Lord's Passion, but even St. John didn't have much hope that Our Lord would rise again.  And then you had Judas being Judas.

Our Lord allowed them to show their weakness precisely to demonstrate the power of His grace to transform them.

This is a form of human respect.  To change actions (or the appearance of beliefs) based on the opinions/needs of others.

I don't agree with that.  He could easily have been motivated by a misguided charity.  He didn't want the Judaizers to abandon their faith or become hardened in their errors, so he played along with them.  You combined opinions and needs of others.  Yes, it's human respect to change actions based on others' opinions, but not necessarily based on their needs.

Here we have the Holy Spirit teaching us through St. Paul (I Corinthians 8):

8 But meat doth not commend us to God. For neither, if we eat, shall we have the more; nor, if we eat not, shall we have the less.  9 But take heed lest perhaps this your liberty become a stumblingblock to the weak.  10 For if a man see him that hath knowledge sit at meat in the idol's temple, shall not his conscience, being weak, be emboldened to eat those things which are sacrificed to idols?

11 And through thy knowledge shall the weak brother perish, for whom Christ hath died?  12 Now when you sin thus against the brethren, and wound their weak conscience, you sin against Christ.  13 Wherefore, if meat scandalize my brother, I will never eat flesh, lest I should scandalize my brother.

One could easily interpret St. Peter's actions as being motivated by the exact same sentiments enunciated here in I Corinithians 8:13 (bolded above).

I think a very important distinction must be made here (which I think was made in the original thread).  Peter was not Pope until after Pentecost, the birth of the Church. 

According to St. Thomas Aquinas, in his commentary on Galatians he says that:

"Apropos of what is said in a certain gloss, namely, that I withstood him as an adversary, the answer is that the Apostle opposed Peter in the exercise of authority, not in his authority of ruling." (p.46)

Therefore, it seems that at the time of this incident, Peter already possessed authority over the Apostles.

Accordins to St. Thomas Aquinas, in his commentary on Galatians he says that:

"Apropos of what is said in a certain gloss, namely, that I withstood him as an adversary, the answer is that the Apostle opposed Peter in the exercise of authority, not in his authority of ruling." (p.46)

Therefore, it seems that at the time of this incident, Peter already possessed authority over the Apostles.

Yes, that was after Pentecost. 

which is how R&R try to falsely leverage this passage.

"blah, blah, blah, R&R!  R&R! blah blah...