You share the sentiments of mid/late 18th century British author Thomas Day. He said:

If there be an object truly ridiculous in nature, it is an American patriot, signing resolutions of independency with the one hand, and with the other brandishing a whip over his affrighted slaves

So for your first question, how were they able to do this? Well, many Founding Fathers knew that the idea of slavery was in direct opposition to their ideas on liberty regarding the Revolution. In fact Jefferson tried to blame the whole slave trade on England in the Declaration of Independence, acknowledging that slavery was a gross violation of natural rights. This language was stricken from the final draft of the document, as it seems the Founding Fathers were not willing to stick to the issue of slavery at the expense of alienating southern patriots from the cause.

(David Armitage "The Declaration of Independence: A Global History")

I can't answer your second question perfectly, but if you look at a some primary sources you'll see that a lot of Founding Fathers spoke out about the issue. Madison specifically refers to slaves as men in the Constitutional Convention:

"We have seen the mere distinction of colour made in the most enlightened period of time, a ground of the most oppressive dominion ever exercised by man over man”

(James Madison, "Notes on the Federal Convention," 1787)

Jefferson also referred to slaves as people, calling it a “cruel war against human nature itself, violating its most sacred rights of life and liberty in the persons of a distant people.”

John Adams spoke harshly of slavery as well:

"Every measure of prudence, therefore, ought to be assumed for the eventual total extirpation of slavery from the United States…I have, through my whole life, held the practice of slavery in…abhorrence."

So, Founding Fathers weren't worried about the hypocrisy of their views because they had independence to worry about. Also at least some of the FF considered slaves people. Hope this helps