Mikey sure was a beautiful kitty. Very elegant in his perfectly fitted tuxedo jacket. My condolences on his passing.
As for Simba, the reason I ask about his lab work is because hyperthyroidism can mask renal disease, and renal disease can sometimes cause hind-end weakness (for a variety of reasons). How well is Simba's hyperT controlled? Is he well hydrated? I'm a bit concerned about his appetite issues.
It probably would be a good idea to take him for another check-up when you can, but in the meantime, would you be able to get a copy of his latest lab work?
"To close your eyes will not ease another's pain." ~ Chinese Proverb
“We must not refuse to see with our eyes what they must endure with their bodies.” ~ Gretchen Wyler