The First Lady was at the Notre Dame rededication, so there was official U.S. representation. I would assume the U.S. Ambassador was there also, but I've not looked it up. It is not unusual for a First Lady to represent the President.
I am missing how Donald Trump's marriage to Marla Maples could be "valid" or how he and Marla were free to marry. Would not the Church have considered him still married to his first wife Ivana (though divorced in 1990) when he married Marla in 1993, two months after the birth of their daughter? And after being divorced from Ivana he marries Melania (baptized Catholic) in a protestant ceremony. Even if Donald and Melania's relationship is non conjugal there is still the public scandal of it all.
Ivana (first wife) was Catholic, and she had been married before, though it seems that this marriage was for the purpose of her securing Austrian nationality. I have found no evidence that she entered this marriage in the Church, but if she did not, that marriage would have been invalid. If she did, however, then it could have been invalid due to its being a sham marriage, but if it were valid in spite of that, this alone would have rendered her unable to marry Donald validly.
If the first marriage was invalid, of course she would have been free to marry, but Ivana and Donald married at the Marble Collegiate Church in NYC, and I seriously doubt she sought a dispensation from canonical form, to which she would have been bound though Donald (a non-Catholic) was not. Assuming this, then the marriage of Ivana and Donald was invalid.
Fast forward to putative marriage #2. Donald and Marla were both non-Catholics, and if not bound by a prior valid marriage, would have been free to marry at least in the natural order. Donald being bound by this marriage would have prevented him from validly marrying Melania, a Catholic, and on top of that, Donald and Melania married in an Episcopal ceremony. Again, I seriously doubt Melania sought dispensation from canonical form.
Ergo, the marriage of Donald and Melania is invalid.
Assuming the marriage of Donald and Melania is invalid (hard to see how it could be otherwise), the question then is "does a person converting to Catholicism, and in an invalid marriage, have to repudiate that marriage publicly, or do they have the option of living in a Josephite fashion --- assuming there is some grave reason to do so --- and simply not discussing the invalidity of their marriage to anyone?". A private citizen might have that luxury, but the President of the United States is far from being a private citizen. Add to this, if he divorced Melania civilly, they are probably under a pretty iron-clad pre-nup that would only act to Melania's advantage. As a practical matter, he may be pretty much stuck with her.
TL:DR, Donald and Marla are probably validly married to one another in the eyes of the Church.