Yes you are pretty much bang on. When the new trains launched there were radio competitions and a sustained period (at least four months) of very frequent (and also pretty annoying) radio adverts up here for the service highlighting more seats, more frequencies and great low fares. I don't think they fully realised that all the increase in capacity was going to do was make it a bit more comfortable.
To think back to around 1998 some of these services ran as two car 158s and now eight car 350s are bursting at the seams some days shows how much growth there has been.
Regarding split working, I wouldn't be so pessimistic. Lots of people, including 100% of those travelling from Liverpool, join and fill up these trains at Wigan and Preston. Split working just gives those people direct journeys, with the same residual joining at Wigan and Preston.
If it does mean advance tickets available from Scotland to Liverpool then of course that could mean an uptick in numbers, but there's no way a 4 carriage 350 from Manchester Airport won't be able to cope until it gets to Wigan or Preston. I'd expect the loading figures for this service to show a very large number of people getting on at Preston, north bound. Given that the train already stops on the WCML at Preston, another way they could manage demand is to remove the Wigan stop, or differentiate its pricing, so that people don't use this long distance service as a local train during the peak. Arguably, a direct Liverpool service may eliminate demand for a Wigan stop anyway, and if not then that portion could stop there.
Looking ahead long into the future, Edinburgh could handle 12 carriage trains, as could Preston. Front train only operation at the other intermediate stations, and there's still the ability to add more capacity in the future if it was really needed.
Personally, Milton Keynes is my preferred limit on a 350 so I don't know how you manage it on a regular basis. You can't beat a 225 in my opinion!