There’s no record of Abraham marrying Hagar. It was clear that he was wrong and used her slave. Having sex with some one is not a marriage ceremony and Hagar was never referred to as his wife.
Getting remarried after the death of a spouse is not polygamy. It’s perfectly legal and even God does not condemn it.
It’s not how many total, it’s how many at a time only due to death. Anything else is adultery.
I do not thiink there is a record of Abram going in to Hagar but once...at any rate, they certainly had no more children together, so that blows the LDS defense of polygamy!