If it's longer trains, won't they have to make the turnaround area bigger to get the whole train on to the opposing track?

My guess is this is the requirement to have a ticket to operate the barriers at the Heathrow Express platforms when transferring between terminals; I think it's recent as part of preparing for direct Crossrail Elizabeth Line services where Oyster/Contactless PAYG operates

I am not sure what you mean? They don't run around at T5C. They go in to the platform and then reverse back to T5A. The platforms are already longer than the current trains.

The shuttle is normally operated as two trains, one on each line going back and forth. They could easily add more carriages to add capacity. If they wanted to increase frequency they do have the facility to add more trains as there is a crossover at both ends and use it directionally like a normal railway would, though.

She came in T3 from HEL on Finnair, bus'ed over to T5A. Her original connection would have been 95 minutes but they put her plan in the stack for like 10 rounds, so she landed 20 minutes late. Her flight left from T5B. The person stopping her was a BA employee. She ended up sneaking around that person has another larger group was getting told the same. She ended up being the last person to board BA285 to SFO.