what temperature will green mustard provide non-viable seeds (or even flowers)

Not sure that I really understand what you want to do?

Mustard planted much later than now will probably only produce leaf, depending on the weather we get. It is unlikely to produce seed and the plants may, or may not, survive the Winter, depending on the frosts we get. If they survive they will grow away in the Spring and flower and seed.

Growth this Autumn should still be sufficient to dig in as green manure.