Trump can’t be the Moshiach because even if be converts, a convert can’t be a leader which the Moshiach is.
In every generation there are people who are fit to be Moshiach, it’s just that Hashem doesn’t feel that the Jewish nation is ready yet. In order to marry the Mashiach you will need to have married him prior to him being anointed. Unlike the past US presidential election when it comes to Mosiach there are many competent candidates and only Hashem knows who will be appointed. By 20 he will be a Talmud Chachom and a leader and it will be hard to get a date with him. There is a better chance of you giving birth to the Moshiach then marrying him.
As far as Polygamy, even when it was permitted it required the first wife’s permission either at the time of the original marriage or prior to the second marriage. I don’t know why you think in the time of the Moshiach it will be re-instituted. Just as it is said who is a rich man? He who is happy with his lot. So too the same can be said regarding a wife, you don’t need a second wife.