The platform was rebuilt west of the bridge in the time of, I think, Network SouthEast. If there was a bay at this time it was not often used as the Greenford trains ran through to either Ealing Broadway or Paddington: it was built or upgraded when it came clear that the increase in train frequency due to Crossrail would mean that the Greenfords would clash with the Crossrail, freight and GWR outer-suburban trains on the Reliefs.
The Up Relief platform was relocated west of the bridge in the early 90s when the reliefs through West Ealing were realigned for higher linespeed in connection with the Heathrow Express.
The bay platform was a Crossrail project, and was because Crossrail was due to gobble up the paths into Paddington the Greenford services used. It was late 2016/early 2017 when it was opened; and the Greenford service has started/terminated there ever since; barring start and end of day.
A fast-charging Battery EMU would be ideal for Greenfords, and probably Marlows.
Windsor, Bourne End and Henley are significantly busier; peak services would easily justify 4/5/6 car Turbo formations if the infrastructure allowed (in the case of Windsor) and there were sufficient units. Indeed the ancillary benefit of the through trains from Bourne End and Henley was that for the peaks it enabled the branch formations to be strengthened without the torturous timetabling it would require these days as self-contained services.
All 3 (well, 2 1/2) justify a 4 car EMU easily at peak time, and if 769 deployment plans remain the same the Henley and the Peak Bourne Ends will get that in the short to medium term. Wiring remains ideal as it would permit 8 car 387 formations to be used for special events such as Henley Regatta or any Royal events at Windsor with only minor platform extensions at the termini - a temporary structure may be viable in these cases.
A 2/3 car BEMU would be woefully inadequate for anything but the between-peaks service on the Windsor or Henley; service frequency is already at the theoretical maximum with absolute minimum turnarounds as it is. There are other places on GWR where the technology maybe useful if proven and the necessary infrastructure can be provided.