He isn't ceasing funding, it's a suspension. Suspension is temporary.
Honestly I'm fine with that. For anyone to have been fine with the investigation and impeachment of Trump, but not be okay with suspending funding pending an investigation into the WHO's operations and potential ineffectiveness in handling a pandemic I would consider them a hypocrite.
I'm sure there's more to the agenda than WHO's ineptitude, but I for one am fine with this call by Trump.
As for opening the country.......
Trump having the "authority" to force states to do his bidding is a little more tricky. 10th Amendment does give rights to the states and some I've seen feel that Trump may withhold federal funding to those states.
Impoundment is limited by the Impoundment Control Act of 1974 where withholding previously appropriated funds must be approved by both the House and the Senate within 45 days. So, no Trump can't withhold federal funding on his own, and good luck getting the House to side with him.
So, Trump is now left with having to deal with the Governors of each respective state that he has previously been on record as stating they either are nice to Trump's administration or they won't get the aid they need. He's painted himself into a corner and this will be very interesting to see how he gets out of it without giving skin off his *** to do it.