Thanks.
I am now using the simple url declaration you advise, without the 'local'.
The epub in question is fixed (it's an "old" book I'm converting in epub).
So it makes sens to use a small subset of the DejaVuSans font.
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However, after doing a more systematic tagging of the mathematical symbols and formulas over a whole chapter, I'm back (partly) to the original problem: some fonts are not shown correctly

I'm tired now. Maybe I'll find the solution another day.
So perhaps I should have gone for the simplest, even if it's not the ideal: use the less specific symbols as possible, looking for other ways to express them. Thus:
- The "∧" can be rendered as "&", which is present in usual fonts.
- The "∨"; has no equivalent but can be printed "or".
- Etc.