if you would have accepted the amount they offered, you could edit your listing to that offer amount.  If they have it on their watch list they will see that.  They will probably get an email from eBay letting them know of the reduction in price.

If someone else sees it at the reduced price and buys it, then you have the sale at an amount you are happy with.  Process the sale and move on.  You have no obligation to sell to any particular buyer.

I doubt you can, as identities are generally hidden.

But if they made an offer, their ID should be available and you should be able to message them.

Failing that log into eBay periodically.