Re: "Cannot stop Spell Checker in Apple Mail"
".... stop the SPell Checker in mid-check and Send the email."
" cannot shut it down because the red X is grayed out. ...."
You might want to side-step the problem, and set the spell-checker to do it's work "as you type"
... or stop using it altogether ... instead of when you send.
In the Mail App:
Try Settings > Composing > Check spelling : You can choose when to spell check, or choose not at all.
(My Mail app (below) is an earlier version, but likely similar to what you see)

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If needed: Apple support's "Settings" article for MacOS 15.5:
Change Composing settings in Mail on Mac - Apple Support
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Else: If you you really want command over the spell-checker while it is checking prior to sending the message,
You could try re-starting your Mac (Apple menu > Restart) wait to log back in ...
Then check whether or not you now see the red X ok, instead of it being greyed out when sending.
(ie: Restarting often restores order if something is not working as expected.)
or
If still not possible, you may want to send feedback to Apple : Feedback - Mail - Apple:
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Mail Support website (info. / support / contact options etc. : Mail - Official Apple Support