Just to let you know that I have confirmed that with OfficeTab enabled, Excel WILL crash when I attempt to open any "untrusted" documents, meaning those that are not marked as trusted, not in a trusted local folder, or are in a remote location, like internet or LAN. It IS indeed related to trusted documents. Disable Office Tabs, and everything goes back to normal. Over time, I have tried on my old and newly purchased notebooks, plus my desktop computer, on Office versions 2016 to 2019, with SAME ISSUES. Tried and true.
And since NOBODY'S DOING ANYTHING ABOUT IT, and it's well passed my refund period, I have simply decided to just trash Office Tab, with a little episode: I gave it to a colleague of mine, and soon he complained of the SAME ISSUES!
As with the problem with Access, I no longer care.
Subject closed.