It’s honestly difficult to predict what’ll happen stock wise with XC. The Avanti 221s would make sense, but we know that sense and the DFT don’t go together.
As for service changes, if Newcastle - Reading ever returns I think it should only run south of York. TPE have already caused enough of a headache with terminating platform capacity in Newcastle when they switched to 802s that don’t fit in the west facing bays, there simply isn’t the capacity for both TPE and XC to be terminating in through platforms every hour at Newcastle, which is what would be required if Newcastle - Reading returned with anything other than 4 car voyagers. The XC Reading service also ran straight behind an LNER service with the same calling pattern in both directions north of Doncaster so it isn’t a huge loss on the ECML part of the route.
No. The 175s will be released first once a few 197s enter service, but whether they go anywhere or not, they will certainly only be used to strengthen an existing fleet and no other DMUs will be freed up as a result, especially not from Northern. 153s and 155s really need to be scrapped so a few TfW 150s and 158s (excluding the fire damaged ones which will be scrapped immediately) may replace those but the rest will go wherever they are needed, which means many to GWR for obvious reasons, but SWR will also need a 158 to replace the one that crashed and realistically they are likely to be split up between as many operators as are in need of more Sprinters.
Apart from the converted ScotRail units all remaining 153s are with TFW and will be replaced by 231s and 756s anyway. There is no pressing need to replace the 155s, certainly no more than any of the other sprinters.
Northern are believed to want to replace the 150/1s, so a transfer of 175s to Northern would partially enable that, depending on where they’d be allocated either directly or through a further internal cascade of 156s or 158s.
SWR will manage fine without an extra unit. Their DMU fleet is the one fleet they’re not short on. If they’re really that desperate for an extra DMU they could have 158889 back from EMR.
With regards to TFW’s 158s, because of the ETCS fitment transferring them elsewhere won’t be an easy task as it will require all drivers even those who already sign 158s to be retrained. As such it makes little sense for any TOC to take those over the 175s unless they have a requirement for ETCS fitted stock.