Hi,
well, I don´t have timeshift but I made a habit of performing a complete system backup once a month using clonezilla.
So before attempting the “older version”-installation I´d have to get hold of a new backup. The newer the better, right? 
Thanks for your suggestion. Better safe than sorry.
Greetings
Rosika 
i use clonezilla as well and a newer backup definitely wouldn’t hurt. i also make quick notes when i do any system work just so i have something to check my memory against in case i have questions later. good luck 
@01101111:
Thanks for your feedback .
Just out of curiosity, because it´s OFFTOPIC:
Do you use timeshift in addition to clonezilla? And if so, does it provide any advantage?
as it is originally your topic (with the solution is still pending) and i believe looking at backup methodologies is at least tangentially relevant, i don’t mind answering briefly
i do use both timeshift and clonezilla. in instances similar to this i believe timeshift offers a quicker response if the change in modemmanager causes some kind of system instability. a restore most often can be started with the system running and even on my hdd takes only about 5 minutes to get me back up and running vs the time it would take to boot into a clonezilla usb and restore from image.
in general i keep up with monthly clonezilla images in case the whole system becomes unusable or the disk itself goes.
Just clone the repository and follow the instructions on how to install from source. The only difference is that you have to change to a different version within the repository. I don’t know how to do that in mercurial repositories, though. In git that would be e. g. git checkout 0.1.0.
Thanks Akito for your suggestions.
I´ll look into that.
Greetings
Rosika