MS when they made office mobile, they made the choice to separate the casual from the professional use. So if the tablet is arm and below 10.1 (and above 7) you can use the core functions for free, if it's over 10.1 you must have an office 365 subscription and there is no way around it.....
unless...
The only thing I can think is that MS office (and other apps) detects the size of the screen by knowing 2 values:
Resolution in pixels and Pixel density or PPI (Pixels per Inch). For example, a 10.1 tablet at 1920x1200 has 224 PPI. For a 9.8 screen at the same resolution, the PPI should be at 231. As you see, by increasing the PPI value, you may trick Office into seeing that you have a smaller screen and so you can probably make it run.
The problem is, there are several ways to change the PPI and it depends if the device is rooted or not, the android version, etc. So my suggestion would be to google about your device/android ver and how to change the default PPI (if it's possible) and try again with the office. Note I'm not sure how this will affect the visual quality of other apps but the only way to find is to try.
After this point, I think it's best to visit a forum that deals with such things like the XDA forums.
Hope it helped.
Edit: you can use this site to see the Screen size/Resolution/PPI relation
https://www.omnicalculator.com/other/pixels-per-inch#how-to-calculate-ppi:-the-theory
Type in the values and then change the screen size, and notice how the PPI value change.
