Bearing in mind the 397s and Mk5s were late going into service if the actual into service date had been known/suspected at the time of order would that of changed the strategy, as in 'OK well they wont be available until ... may as well order 80x and wait for the current production runs to finish'
Having multiple smaller fleets (185, 397, 802, Mk5) must increase the complexity of operations and costs, more training, different maintenance requirements, cant swap units around as easily. Assuming TPE had been able to purchase just 802s then current operations would be much simpler, they could shuttle units between ECML and WCML operations, and all their drivers would just need to know one type of unit.
I have only had one short Manchester - Preston run on a 397 it was OK, seats were comfortable, lighting was adequate but not intrusive, but surely that is down to the way the units are fitted out, could be an 802 or 397 body shell, its not going to matter. I dont think the issue is passenger enviroment, I think its more 'under the hood' stuff. Same with the Mk5s as a passenger they are an improvement on 185 for long journeys, doors at the ends, no engine under the floor, seats on a par with 185s, but I gather staff are not impressed, there have been a lot of issues with them.