As I say, the EU guidelines say no trade talks until all the commitments made in December have been legally signed up to. So if the UK wants to talk trade and wants to avoid no deal and wants to avoid NI staying in the CU/SM it has to come up with something on (a) and (b). I suspect that need be no more than a commitment to keep working on them. It is a negotiating stance to say you need to talk trade first. It is not essential in itself.