We often try and impose what occurs during the Church Age (after Pentecost) on earlier periods. When Jesus Christ was carrying out His ministry He only went to the Jews (with a few exceptions). He preached the Gospel of the Kingdom and it was only after Pentecost that the Gospel of Grace was taught and extended to the Gentiles. Yet there is a large school of thought within Christianity that thinks there is no difference in either period.
If the latter were true where were the pre-Crucifixion believers? If their sins had already been paid for why would we need a Crucifixion in the first place.
Eph. 4:9 Now that He Ascended, what is it but that he also descended first into the lower parts of the earth?
When Jesus was crucified he told the thief that believed, "today you will be in paradise". He did not say you will be with me and my Father in Heaven. We see the same confusion when Matt. 16:18 and "the gates of Hell" are misinterpreted to benefit a large institution.
I concur. Notice even Matt. 15:24, "But He answered and said, 'I was sent only to the lost sheep of the house of Israel.'" Yet, He had mercy on the Gentile woman & her daughter. But, you are correct, we had not yet been grafted into share Messiah.
I might offer this small adjustment to your remark about Pentecost. It seems the Gentiles inclusion actually occurred at the shedding of His blood. Eph. 2:11ff, "...remember that you were at that time separate from Christ, excluded from the commenwealth of Israel, and strangers to the covenants of promise, having no hope and without God in the world. But NOW in Christ Jesus you who FORMERLY were far off have been brought near by the BLOOD of Christ." It was during the 40 days following His resurrection that the 11 were told to take His message all over the world. It became very obvious at Pentecost with Peter's remarks.
Nevertheless, the "timeline" effect you pointed out with the "gates" & "Abraham's bosom" is a very important concept and ought to affect our hermeneutic all through the Scriptures. The result would be a Gospel that is significantly clearer and absent the muddled message of "Grace plus just a little Law" we now hear from many quarters. No more "Golden Rule", no more "Beatitudes for Living Right", no more "take your sacrifice to the priest" blended with the real message of grace.
It is no wonder Rome cannot deliver a true message with their attempts to amalgamate the four Gospels into "Christian Living". Many of the "protesters" cannot recognize what you have identified.