In general, the Bristol/Bath railway path is quieter in the winter, that said it's still busy at peak commuting times so you're rarely alone on the path, and the times you'll be riding will be peak commuting times so you'll rarely be alone in the dark on the path.
Lighting is a tricky subject, the path isn't wide and you'll quickly tire of being dazzled by some very bright lights, I'd go for lights that have a very narrow focussed beam for use on the path, just enough to illuminate the surface ahead of you, and a wider beam for when there's no-one else around or when you ride on the road. Some sections of the path have excellent lighting, others not so much, you will need lights to see with , so don't rely on pound shop blinkies.
Because the path covers three different local authorities, winter treatment varies. In general it's rare for the path to be gritted fully, sometimes busy sections are gritted in Bristol, sometimes in South Gloucestershire but seldom does it happen in Bath.
Currently, we're approaching autumn and any day soon the path will become a mess, expect to be covered in it on wet days.