I'm not too sure what part of Matthew 23 you are referring to.
The sermon was only to the temple staff and it was at Passover so it is quite possible that Saul was in town. That group is a remnant of the temple staff that caused God to send in Neb and take them into exile in Babylon. They are also the ones being front and center as far as who was to not be told about the vision on the mountain with Christ, Moses and Elias. God intentionally kept that from them so they would fulfill the prophecies.
When Jesus said, 'It is finished.' moments before He died on the cross it was a reference to all things to do with the bruise to the heel from Ge:3:15. The cross was in the middle of the 70th week, 3 1/2 more years would be needed to fulfill that prophecy. The last event done for the Jews alone was making Gentile food clean and that was in preparation to the scattering from the prophecy in De:4:30. The preaching Peter did in Acts:10 was the first act for the people that would be involved in the 'latter days' from that prophecy.
When Saul saw Stephen being killed it was before the end of the 70 week and that is known to be associated with the heel bruise while the 'latter days' is about the prophecy and fulfillment of the bruise to Satan's head.
If nothing else Saul was sincere in his devotion to God, however what was given to the Jews was applicable to the heel bruise, all NT teaching is focused on the ones that will face judgment, namely all the nations on earth. When Neb was sent in it was as a servant of God so that puts Neb in line for a reward, that also applies to all Gentile Nations between Neb and the last Gentile king to rule over Jerusalem. That will be found in the two witnesses as the ones that kill them are angelic beings that make a return from when they ruled on earth before the flood. Any sins between the cross and the food vision in Acts:10 is a reference to the 'overspreading of abominations' that would take place in that 3 1/2 year period. From 33AD to 70AD it was when the Apostles stayed in Jerusalem and taught the same message that Paul and the Beloved Disciple were teaching in the Nations. What they started has to be completed before the judging of the Nations from Jer:25 can begin. Since there is only one time nations are judged all OT prophecy about that sort of event is a reference to that singular event.
I have no problem with Paul as with any of the other apostles. I do think we need to acknowledge that Paul never saw the resurrected Jesus. He did have an experience on the road to Damascus. He did have a mystical experience.
The conversion that Saul went through fits in with the prophecy from De:4:30 and the sincere searching in the period of time known as the 'latter days' as they started in the middle of Acts:10 and that will last until the day the two witnesses are resurrected. That is also the day the Jews from Matthew:23 will hail Jesus as being the Messiah promised by God in the OT Prophecies. The literal resurrection of the whole House of Israel as written about in Eze:37 is when the ones at that sermon will fulfill this prophecy.
M't:23:37-39:
O Jerusalem,
Jerusalem,
thou that killest the prophets,
and stonest them which are sent unto thee,
how often would I have gathered thy children together,
even as a hen gathereth her chickens under her wings,
and ye would not!
Behold,
your house is left unto you desolate.
For I say unto you,
Ye shall not see me henceforth,
till ye shall say,
Blessed is he that cometh in the name of the Lord.
That wasn't Paul. The only one taken to the 3rd heaven is the writer of Hebrews:12 as the chastisement that takes place in that chapter is said to be at Mount Sion.That is the same location as the Great White Throne in Re:2 and New Jerusalem in Re:21 and 22. The only other person shown that was the Beloved Disciple when she wrote the Book of Revelations.
There are 3 versions to Saul's conversion,by himself, by those he was with, and how the Holy Spirit saw it.
Ac:9:3-7:
And as he journeyed,
he came near Damascus:
and suddenly there shined round about him a light from heaven:
And he fell to the earth,
and heard a voice saying unto him,
Saul,
Saul,
why persecutest thou me?
And he said,
Who art thou,
Lord?
And the Lord said,
I am Jesus whom thou persecutest:
it is hard for thee to kick against the pricks.
And he trembling and astonished said,
Lord,
what wilt thou have me to do?
And the Lord said unto him,
Arise,
and go into the city,
and it shall be told thee what thou must do.
And the men which journeyed with him stood speechless,
hearing a voice,
but seeing no man.
Ac:22:6-10:
And it came to pass,
that,
as I made my journey,
and was come nigh unto Damascus about noon,
suddenly there shone from heaven a great light round about me.
And I fell unto the ground,
and heard a voice saying unto me,
Saul,
Saul,
why persecutest thou me?
And I answered,
Who art thou,
Lord?
And he said unto me,
I am Jesus of Nazareth,
whom thou persecutest.
And they that were with me saw indeed the light,
and were afraid;
but they heard not the voice of him that spake to me.
And I said,
What shall I do,
Lord?
And the Lord said unto me,
Arise,
and go into Damascus;
and there it shall be told thee of all things which are appointed for thee to do.
Heb:12:22:
But ye are come unto mount Sion,
and unto the city of the living God,
the heavenly Jerusalem,
and to an innumerable company of angels,
Heb:12:23:
To the general assembly and church of the firstborn,
which are written in heaven,
and to God the Judge of all,
and to the spirits of just men made perfect,
He framed that experience by talking about a resurrected God to his Greco-Roman audience.
If anybody pulled a fast one it would have been at this event.
Ac:17:22:
Then Paul stood in the midst of Mars' hill,
and said,
Ye men of Athens,
I perceive that in all things ye are too superstitious.
Ac:17:23:
For as I passed by,
and beheld your devotions,
I found an altar with this inscription,
TO THE UNKNOWN GOD.
Whom therefore ye ignorantly worship,
him declare I unto you.
Once again its all part of the same NT. I think the most important point here is how the New Testament replaces the Old.
Hebrews 8:13
It doesn't replace it, it has more information on the two bruises from Ge:3:15. The 4 Gospels give the details on how the bruise to the heel was completed and the rest of the NT is a guide to the people that will face the last judgment as a few of them will be living witnesses rather than being a resurrected person. Ge:1-3 and Re:20-22 are a mini Bible as far as who is God and where are we.
That covenant isn't made with anybody but the people that are alive on the earth at the end of the day the 7th trump sounds.