All I can do is present what the Word says. And explain where there seem to be differences, which you seem not willing to accept.
Peter said in Acts 2:38 "...be baptized every one of you in the name of Jesus Christ for the remission of sins, those who believe in Jesus will be water baptized in His name and receive remission of sin.
Without a doubt that crowd was unique in their sins. Just like the Pharisees who SAW the miracles (done by the power of the Holy Spirit) but they claimed the miracles of Jesus were done by Beelzebub. Basically ascribing the power of the Holy Spirit to Satan himself.
iow, they were so negative to the truth, that NOTHING would convince them of truth. That is why Jesus said they wouldn't be forgiven. He knew they would not repent. Just like many in the Tribulation who will not repent. Rev 16.
Peter also tells the Gentiles that through Jesus' name whosoever believeth in him shall receive remission of sins. (Acts 10:43) And Peter commands them to be water baptized in the name of the Lord.
As I've shown from that verse, the KEY is "everyone who BELIEVES IN HIM". That's how people are saved. You are conflating Acts 2:38 with 10:43 and trying to make 10:43 say something OTHER than it does.
As for the thief, he died under the OT and was not required to obey the NT commands.
It is WRONG to think that salvation was by a different method than in the NT. Just follow Paul's own preaching. He "proved from the Scriptures" that Jesus is the Christ. What Scriptures? The OT, of course.
There is NO difference between OT and NT salvation. It's all the same.
The NT conditions began after the Holy Ghost was poured out on the Day of Pentecost.
You need to read what Paul preached. Everything came from the OT Scriptures. There is NO difference in how to be saved.
And, remember that Paul said he WASN'T called to baptize but to preach the gospel.
Rom 1:16 - For I am not ashamed of the gospel, because it is the power of God that brings salvation to everyone who believes: first to the Jew, then to the Gentile.
Not ONCE in Paul's preaching did he claim that salvation required water baptism.