These are "character building" projects.
Yes, and they can be "muscle building" as well...
I just framed an already stretched canvas that was out of square in a floater last week.
We do a lot of canvases in floaters, but most of them are square.
Whenever I get an already stretched canvas to frame, the first thing that I do is check the size and check for square.
I measure top, bottom and middle on the length and width for size.
Then I measure diagonally to see if it's square.
When I see an issue, I put a metal corner square on it or sometimes I just use a sheet of Fome Cor to see how far out it is.
The one that I did last week was about 30" x 44".
It was out of square by about 3/8" and it was to be fitted into a floater with NO GAP around it.
We have a designer that likes to have no gap and flush to the front so everything has to be perfect.
I tell the client that it's out of square, get permission to re-stretch and I re-stretch it slightly smaller to make it square.
I only screw the canvas into the floaters from behind with countersunk screws, so I'm not sure about what the OP meant about "glued and screwed" into the floater.
I would never glue a canvas into a floater.