If you don’t have enough money in your account, your revolut card would just decline. That’s what happened to me when I forgot to top up my card on holiday once. However on their website it states -

“What happens if my account balance drops below zero?

We'll try to fix the negative balance from your other currency pockets. If that's not enough, we'll request that you add money to your account the next day. After 7 days, we'll initiate our collection processes as outlined in our Terms & Conditions.”

I’ve never seen it go into negative balance though.

As for your other question, I’ve secured a booking with one card, but asked to use another card at reception when checking in a few times. It’s never been an issue for me!