Why should the RMT even be in dispute? If the dispute is about safety as the RMT keep telling us, surely it's for ASLEF to agree a safe method of operation with Northern.
The RMT are in dispute because it's their members' jobs that are potentially at risk, although of course safety concerns form a key part of the dispute. I'm pretty certain that ASLEF will end up failing to agree in due course with a different agenda (but hopefully the same ultimate objective), but - wisely or otherwise - the RMT have chosen to preempt those discussions.
Northern claim they are willing to discuss which routes get DCO with the unions but the RMT refuse to discuss it. Maybe the RMT should try telling Northern they'd be happier with a DCO Manchester-Marple service than a DCO Manchester Airport-Barrow service if that's the case.
To do so would undermine their position that no extension of DOO is acceptable, although I don't entirely disagree with what you suggest. "Less unhappy" rather than "happier", perhaps.
The government aren't going to give in just because the RMT don't like it and call at a strike as the first resort. Remember Rail North (including Labour controlled TfGM and Metro) signed off the DCO requirement and it's rumoured the agreement they signed off on is vastly favourable to the original proposed by DfT.
If the government's objectives were (a) transparent and (b) made sense, then we might all be able to make some progress. As it is, the government's pretending that it's all Northern's doing, Northern seem reluctant to highlight the DfT's role and the RMT won't stop going on about German basketcases. I just can't correlate the statements being made by the industry and the government with the RSSB's reports with an emphasis on cost savings. If it's not about cost savings, what's the point of it all? If it's about avoiding the hypothetical 0.25% of services being cancelled (as you suggested earlier), then it's not worked as I'm sure that far more services have been cancelled this year than would've been cancelled in my lifetime otherwise. If it's about "improving customer service", then it just seems flawed and a poor alternative to just managing and supporting the existing staff properly.