Your post didn't directly fix my problem (unable to open the QB file simultaneously on two computers, even though the file was in multi-user mode), but it guided me to a fix.

See #5 in the following link.  

https://quickbooks.intuit.com/learn-support/en-us/help-article/product-setup/nd-files-quickbooks-des...

1 for Hosting On; 2 for Hosting Off

Both of my .ND files were set to 2, not 1.  I manually edited the files in Notepad (changing the 2 to 1).  Next time I had no trouble opening the QB files from both computers.