Electronic Signatures in Excel

Hi, Having difficulties -- and looking for a proper fix please.

I have an Excel workbook saves as a template. It uses multiple calculations and conditional formatting to ensure that the spreadsheet is completed properly. Within this, on one sheet, is a 'Signature Line', which will (or should) indicate that the final saved document has been completed and "signed off".

In practice, this is not working for the end-user. We have the following issues:

1. The Electronic Signature is calculated for the whole workbook, rather than the individual sheet. Whilst this is really annoying, I can 'fudge' a workaround for this (although it is a really ugly 'kludge').
2. The standard, default, method of opening a spreadsheet involves the automatic recalculation of all formulas (and thus destroying integrity), irrespective of whether the cell contents have changed. This has the effect of invalidating any previously applied signature. 
3. To counter the problems in (2), I have tried changing the template Calculation Options to 'Manual', and re-saving the template. Re-opening either the template or a .xlsx workbook saved from the template appears to remember the 'Manual Calculation' setting -- however, this (currently) does not fix the problem, and I still am getting an 'Invalid signature' message -- despite being mentioned as a fix in multiple online support pages.
4. I don't know why, but occasionally this setting to 'Manual' appears to revert to 'Automatic' on occasions. I have not discovered what causes this.
5. I cannot use any VBA code to 'fix' this problem, because VBA is declared by my organisation to be a security issue, and is globally disabled.
6. Likewise, I cannot disable the full internal recalculation behaviour with a registry key change (eg: FullCalcOnLoadOldFile = 0)
7. As a side issue (but currently unable to test), I also need a solution to the problem described here: Excel digital signatures - Microsoft Community Whilst a certificate may expire, and not be valid for signing new documents, it should still verify that a signature on an historical document has a valid signature based upon the date it was originally signed (and not today's date).

Some of the community posts documenting this problem are many years old -- and none appear to have been properly and comprehensively answered anywhere.

• Is there a "fix" that I can apply that will actually work (for users other than me)?
• Are there plans to properly patch Excel so that Electronic Signatures will work and validate as expected in future?

Many thanks.

Resources referred to (all without success):

• Add or remove a digital signature for Microsoft 365 files - Microsoft Support
• Digital Signatures become invalid after upgrading to MS Office 2010 - Microsoft Community
• I have used a digital signature in a excel file, However when I got - Microsoft Community
• Invalid signature - Microsoft Community
• Excel Workbook File Digital Signature Errors - Microsoft Community
• M365 excel invalid digital signature - Microsoft Community
• Document contains invalid signatures error if opening digitally signed in newer version Excel file - Microsoft 365 Apps | Microsoft Learn
•