The Chat Moss stopper cannot be worked by 6-car Class 323 formations because most of the platforms are not long enough. 6-car Class 331 formations could be used, because these units have Selective Door Opening, but I doubt that this route will get priority for the limited number of these units. 3-car 323s are the most likely traction.
Whether or not capacity on the Preston - Bolton - Manchester line becomes problematic in future depends on the extent to which demand recovers post-pandemic. The line will have 2tph of 6-car Class 331 Airport - Blackpool services and 1tph of 5-car Class 397 Airport - Scotland services in addition to the Airport - Cumbria service. The Victoria - Preston service, which will be withdrawn, was never longer than a 4x20m Class 319 pre-pandemic, so a 3x24m Class 195 might still be sufficient for the Cumbria service. If not, 4-car or 5-car 195 formations might be possibilities.
I doubt that Northern would consider splitting/joining units at Preston, because additional drivers would be needed for the shunting operation. IIRC this was last done years ago before Bolton/Blackpool electrification, when 6-car Class 185s used to split into Cumbria and Blackpool portions at Preston.