I am able to complete the challenge to earn the 500 points, but it is not working as expected.
When I added just Herman, Lynda to the permission set Temporary Account Edit, when run the flow, I am consistently getting error Error Occurred: The permission set must be pre-assigned to the user before being activated 
I added a second user, System Administrator, and now the flow worked with two users in the permission set, but when I logged in as Herman, Lynda, I don't see the Account name is editable, so the session based permision set logic didn't really worked.
I removed the System Administrator, and added a Platform user account, but the flow gave same error "Error Occurred: The permission set must be pre-assigned to the user before being activated"
I changed the user Herman, Lynda to Standard User (that has CRUD access to account), and added to the Temporary Account Edit permission set, and logged out,a nd logged back in, and ran the flow, and again gave same error.
I tried with flow builder and cloud flow designer, and both are behaving same.