As you likely know, the we should was found in no manuscripts and was inserted by faith from the translaters...
Paul's commission to go to the Gentiles was already established by God...
kai gnontes ten charin ten dotheisan moi Iakovos kai Kefas kai Ioannes oi dokountes styloi einai dexias edokan emou kai barnaba koinonias ina emeis [men] eis ta ethne autoi de eis ten peritomenword by word, hyphenating what is a single word in the original:
and knowing the grace the given me Jacob and Kephas and Iohann the esteemed pillars are right-hand gave me and Barnabas fellowship so-that we [on-one-hand] to the gentiles they on-the-other-hand into the circumsized
"Men" is a mere particle: "men folld. by de in the correlative clause or clauses, on the one hand, on the other hand" (see at Lidell, Scott link above, part II). The text without "men" is incomplete grammatically (the proper grammar is "men ... de"), but the meaning is not altered: the natural apostles establish "koinonia", communion or fellowship, with Peter and go on their missions.
The modality "should" is not the missing "men" anyway. It is expressed by "ina". Compare the same word used 6 times in Galatians 2, starting with "that they might bring us into bondage" in verse 4.
Thank you for the opportunity to research this. So, who teaches that "should" in its imperative modal sense was inserted? The original, and Lidell-Scott show that a particle is missing, but the modality is present. Another thing that is missing in all Greek versions is the verb "go", and that indeed is extrapolated by all translators.
Paul's commission to go to the Gentiles was already established by God
What the scripture tells (Gal. 2, Acts 10f) us is that it was established by St. Peter and possibly in congress with James and John, who, of course, were all lead by the Holy ghost in all they did. The impetus to go to the Gentiles originated from Jesus at the parable of the guests at the wedding and the workers at the vinyard; it was given the natural apostles as the Great Commission, but St. Paul had to be adopted into the apostolic college and be given the mission at that later time.