If I send multiple zero value transactions from address
A, containing several messages to broker a deal, is it safe to (re)use that address later on for a payment (to settle the deal)?
Zero value transactions don't need to be signed but you can sign them, either by mistake or by using a buggy library and in this case: it is unsafe to re-use the same address.
I can't find the reference right now, but I think that a bug like that was once reported with js library. (it was probably with the old js library)