Latest I've heard is that the trials of the 350/2s on the Glossop line showed they could not keep up to timings so will not be suitable. The timings are very tight on this line, and the 323s remain superior for acceleration on a line with stations very close together.
Network Rail are apparently willing to move the signal on the bay platform of Stoke-on-Trent, so that a 350/2 can still give the driver a clear view of the signal.
So it's a mixed bag in terms of 350/2 suitability. I cannot see Northern taking on a few whilst keeping some 323s as well, it's not going to happen, it's another layer of complexity into an already complex operation.
The 2031 deadline is for ETCS on the WCML North. So by then 323s will not be able to be used on routes controlled by WCML signalling. Lines such as the Glossop line will be fine still for 323s past 2031, until the 323s are replaced by new rolling stock.