The answer is in your own words there, that the people getting on and off at Preston will not be so numerous, as they will just continue sitting on the 4 carriage section going to Liverpool. If the Manchester portion is busy, it won't be as busy as you think. The number of people who join Transpennine Express services to and from Scotland at Preston is generally a large proportion of the train.
Preston is the key interchange for people going to and from Liverpool. There isn't currently a decent way of counting these passengers due to split ticketing (advance tickets from Preston and an open return from Liverpool to Preston being cost effective). Also affects some other places like Warrington, Wigan. This was spoken about by the official Scottish response to the ITT which has requested direct Liverpool services.
I think you've missed the point - 8 coaches are already full from Manchester!