NOTATIO

The screenshot with the red arrow added has a unison cello line in the bass clef. The other screenshot is from the next system with cellos divided into three, the top stave in treble clef, the other two in bass. Should or should not the cautionary clef change be shown as arrowed.

Tricky case. I would avoid showing the clef by writing the unison material on the 2nd Cello staff for at least that system.

This would be fine in the part if there is a system break in the same place, but if not, then you have a whole other problem.

Yes, tricky.
The best solution is not to make the system break precisely there.
If you must (as it is now), I believe you need to make one measure more divided already in 3 with correct "cleffing".

EDIT: Oh, I remember now, I had exactly the same problem once. As far as I remember I used both cautionary clefs, firs G than F, they were attached in Finale as Expressions, like this:
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I don't know if it is covered by any rule, but no complains from performers.

I think I rather like the idea of the double clef. I'll give that further thought.

Thank you both for the suggestions and comments.

I would move the measure that contains the cautionary clef to the next system so that the divisi starts with the measure that contains the cautionary clef instead of the following one.

OCTO.s second solution is creative but non-standard (and possibly leading to consternation and questions) and, in this case, easily avoided.