Acts 4:12 IS dealing with Messianic salvation, which is eternal salvation and the same salvation that follows in Acts 5:31 and 17:11. Peter was speaking to the Sanhedrin and telling them the necessity of salvation in Christ alone, and in no other (dei sōthēnai).
Peter's sermons were not sermons on eternal salvation. He was dealing with Jesus the Christ as the Messiah (King, Annointed One).
The message of the kingdom and the message of eternal salvation by grace through faith are two separate messages. If you keep trying to combine them you are going to place a great number of contradictions upon Scripture, which can not exist.
Here is another way that you can tell this isn't dealing with eternal salvation, becuase repentance is required, which is not required for eternal salvation by grace through faith. Only faith is required (Ephesians 2:8-9, Acts 16:30-31, Romans 4-5).
And on top of that it was a call for national repentance not individual repentance.
Any way you draw it up the two messages are distinct and different. One is delievered to dead men in trespasses and sins and the other message is delievered to spiritually alive men.
Spiritually dead men couldn't even have understood the message of the kingdom, because it was a spiritual message. The only message a spiritually dead man can entertain is the Christ died for him according to Scriptures.
Once the person is made alive spiritually then and only then can they even understand the message of the kingdom.
That's why you have to keep the messages separate, because they are dealing with different things.