You said:
I can't agree with your reasoning here. Regarding Mark 16:16, why would Jesus mention baptism in the second clause? Why would an unbeliever be baptized? Likewise in the statement by Paul you cited there would be no expectation of perserverance in well-doing by those who are not persuaded by the truth but are persuaded by unrighteousness.You're too close to a works based righteousness here. This verse strikes me as similar to Mark 16:16:
He who believes and is baptized will be saved; but he who does not believe will be condemned.
The second clause indicates what the bottom line for salvation is: belief. The first clause indicates that baptizm and salvation naturally go together. So also in the verse from Romans, the latter clause indicates the bottom line: those who are not persuaded by the truth.
A physician might inform a patient of a diagnosis of a hidden brain cancer that will be fatal if not removed and say "if you trust me I can remove this tumor safely and save your life, but if you do not believe me, you will die". Failure to mention surgery in the second clause does not mean it is unnecessary.