2.) Can old MS Office and Office 365 be used in parallel?
Of course there are no problems here, even if both are used at the same time / in parallel!
3.) Docx, XlsX, ... open documents with old MS Office!
There is also the compatibility package for old MS Office 97 - 2003, which allows you to open docx files, of course there can sometimes be errors in the interpretation of the display, but it has always worked well for me.
Quite simply it is in MS-Excel Euro or US $ sum to calculate, here a simple example Formulas and functions are the strongest tool in Excel. Even complicated
Here are some of the differences between Office 2016 and Office 365, with Microsoft Office 2016 and Office 365 almost the same product There is also the