“The RCC does NOT recognize the Mormon baptism as valid, IIRC.”
Correct; the Catholic Church only recognizes baptism done “in th Name of the Father, and of the Son, and of the Holy Spirit (or Holy Ghost).” That would include almost all Christian churches, groups, denominations, but not the Mormons.
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The LDS Church does not recognize any other baptism, but their own. The baptism is done in the name of the Father, and of the Son, and of the Holy Ghost. It is not expected that another religion honor the LDS ordinance of baptism if an LDS member choses to join another religion. It would be reasonable that the newly converted would subject himself to the requirements and ordinances of his new faith.
The problem, then, would be a serious difference of doctrine on the nature of the Trinity.