will my existing Adobe Acrobat Pro 2017 run under Windows 11 (runs fine, under Windows 10)

I can't guarantee Acrobat 2017 will run right on Win11 as it's 5 years out of date.  It might work, it might not.  However I know for sure that current Acrobat DC Pro works with Win 11. 

https://helpx.adobe.com/acrobat/system-requirements.html

After installing new OS, you'll likely need to reinstall Acrobat.

https://helpx.adobe.com/download-install/kb/acrobat-2017-downloads.html

FREE Acrobat Reader -- view, comment & sign PDF.
https://acrobat.adobe.com/us/en/acrobat/pdf-reader.html

Paid Acrobat Pro DC -- the complete solution for creating and saving PDF.
https://acrobat.adobe.com/us/en/acrobat/acrobat-pro.html

yes, it should work without problem.

and windows has a compatibility mode, if needed. so, you can run any exe under different os settings.

right click the exe > click properties > click compatibility > tick run in compatibility mode > select the needed os.

(note: the above screenshot doesn't show win 10 because my os is win 10.)