I was asked indirectly for a source that the plate breaking minhag is related to the importance of keeping the tenaim.
The source is the sefer Otzar Dinim U’minhagim by Yehuda Dovid Eisenstein , originally printed in 1917. It can be found in any good bookstore. Page 438,
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The minhag is quite old, as there later developed chassidishe interpretations by the Besht and Rabi Nachman of Breslov. The original explanation appears to be much older.
Why the mothers? I have some thoughts, but anyone’s guess is as good as mine.