@Tracy405 Yes, CS seems to be suffering from the firing of 1900 employees a couple weeks ago. Maybe try them through their Twitter account, it's said to get better results.
In the meantime, I'd message these guests again, telling them that if they don't respond to the request for the extra guest fee, you'll unfortunately have to cancel their reservation. That might put a fire under their butts. And some guests simply don't have their notifications turned on- you could try texting them to say you've sent them a message they need to respond to ASAP, so to please check their Airbnb Inbox.
And if you have IB on, I'd turn it off. With CS difficult to contact, the more control you have over bookings, the better.