Recently on my business trip to London, the train was full + standing both ways. 5 car 222 down (peak hour 7am train) and 5 car 180 back (peak hour 5:30pm train). The 5 car came all the way from Lincoln before picking up at Nottingham!
Perhaps time the diagrams so the longer units are doing the peak pickups from London. That said, the EMR fleet doesn't have a huge amount of flexibility in this regard.
Perhaps this belongs in speculative, but:
-Use 222 cab cars as 2/3 car units for 'long distance' regional, then utilise middle cars to lengthen trains? I would like to see some 222 crowdbusters stay on the MML, even with the new Hitachi fleet.
Reserved seats should help, in theory. My observations on XC, TPE and LNER suggest a very high proportion of reserved seats are never taken up - but that's yet another story.
In this case, reserved seats decrease train capacity. I've walked past many seats on XC marked as being reserved from a couple of stops after, just to find they never get occupied.
Trains aren't aeroplanes, making stops mid journey fundamentally changes the dynamics. Plus, you have a mixture of people booking in advance, as well as directly before departure, even on long distance routes.
In my opinion, reserved seats are a perk you should pay for. Certainly shouldn't have them be mandatory on certain advance fares. All this does is fill up the train with "reservations" which aren't used 50% of the time. I do it all the time - find a more convenient seat and just sit in that, the majority of the time I don't use my reserved seat, even if I have one.
In terms of spreading people throughout the train:
-Often on TPE Manchester to Cleethorpes, my reserved seat ends up being in the part of the train nearest the entrance at Piccadilly, so actively worsens the spread of passengers throughout the train.
-This is where some legwork has to be done by announcements and guards to encourage people to move to the other units. Frequent travellers eventually learn this anyway.
-Station boards can help point people towards quieter parts of the train.
-Stations need to adapt their flows in the longer term to better spread passengers along a train.