Portable older version of MS Excel - where to find?

I'm aware that this may be a bit of an X-Y problem, so I'll start from the beginning.

The latest update to MS Office at my corporate workplace has unfortunately broken my Excel application. It relies heavily on VBA, spread over multiple files. Several are .xlam files, which have been split up simply for convenience so the user does not have to load ALL code in one go, but may load files separately according to which section of the application they want to use.

The error reads:

"Excel cannot open the file Portal.xlsm because the file format or file extension is not valid. Verify that the file has not been corrupted and that the extension matches the format of the file"

I know for a fact that there is nothing wrong with the file. It is not corrputed, because I managed to e-mail the file to myself at home. It opened using my home Excel, and I was able to diagnose precisely which line was giving the error. When the (non-critical) line was removed, the file opened properly with my corporate environment.

Out of interest, the offending line was this:

'Required functions to test the state of the shift key (for developer to pause Launcher)
Private Declare Function GetKeyState Lib "user32" (ByVal nVirtKey As Long) As Integer

I do not know exactly why this GetKeyState function would have caused the entire Excel file to fail to open - so if anyone has any ideas, I'd like to hear them (I'm aware that I should repeat this post on StackOverflow too)

However, although this primary file was now working, a second problem arose. The XLAM files were unable to be loaded. More mysteriously, when I attempted to open them on my HOME computer, they were not recognised. At home I have Excel 2019 (Version 2110). At work I have Excel for MS Office 365.

So what I'm now thinking is that if I can open the xlam files somehow, I can retrieve the code and copy/paste the modules into the primary file. As it was before I decided to split them up. The only problem is that since a mysterious security update, I am unable to open these xlam files.

Which brings me to my question: if I can use an older version of Excel, I could open these xlam files - purely in order to view the VBA modules - and then copy them as described here.

Could anyone please suggest a place where I could access older versions of MS Excel? Preferably Portable, because I would rather avoid the hassle of re-installing. Thank you.

NOTE I work for a very large government department, and have zero admin rights over my corporate network. No chance of changing any registry configuration there!

Deployment by Office 365 Admin center is only for Web Add-in, the new generation, so your add-in of .xlam cannot not indeed be deployed any more.

I don't know of a portable Office version, but I know from where you could download old versions of Office. These could be installed inside a virtual machine, to avoid deranging your working environment.

You could download several Office versions from the website TechBench by WZT:

The images are downloaded from the Microsoft website (even if officially these versions do not exist there). There are no serial numbers to be found in this website (it's not a hacker site), but the trial period will be enough for your purposes.