I phrased that clumsily. What I meant was that Trump is not in a position where he would be living conjugally with a woman who is not his wife in the Eyes of God.
Ivana is deceased, so whether her marriage to Trump was valid or not, is beside the point. Marla is still living, so (a) if Trump and Ivana were validly married, then there was no marriage to Marla, or (b) if Trump and Ivana were not validly married (Ivana was Catholic), then both Marla and Trump were free to marry one another, neither was Catholic, therefore their marriage would be presumed valid.
If (a) were the case, then Melania and Trump's marriage could easily be convalidated by the Church (Melania is Catholic), as Ivana is deceased, and if (b) were the case, it really wouldn't matter, as I seriously doubt Melania and Trump are intimate anymore (a Josephite marriage is a celibate one), and they seem to live separate lives (while Trump was President, Melania and Barron lived with her parents in suburban Maryland).
The Catholic Church, "true" or otherwise, would have no need to validate Trump's marriage to Marla, as neither one was Catholic and it was therefore putatively valid, if he and Ivana were not married in the Church (see above). So far as I am aware, Marla had no previous marriages.
TLDR, if Trump became Catholic, there would be no obstacle to his receiving confirmation, the Eucharist, or the sacrament of penance, as long as he did not live conjugally with Melania, in the event his marriage to Melania couldn't be convalidated.