Hi there
Assuming you’re correct about ‘broadening’ having both dynamic and tempo meanings (I don’t think it has a universally accepted dynamic meaning), there is still an ambiguity: did the composer intend both meanings at once?
It’s always better to write with a bit too much clarity, rather than not enough.
It’s been my experience that sometimes the placement and styling of a term can resolve ambiguity: if the word ‘breit’ appears in italics under a violin stave, it’s not a tempo marking. So the modern style (as practiced by Dorico) is helpful in keeping different classes of text styled and placed differently. I would always separate dim. and rit.
J