6. Is that the Severn Beach line?
Yes ... though it has changed a bit since then.
3. is Portsmouth and Brighton to Warminster (and Dilton Marsh), pre-dating the May 2022 removal of the through GWR▸ Brighton to Bristol/Great Malvern services (and also the end of on-board catering, which I learn from this Forum was from April 1st 2022).
Yes
5: Looks like Marlow via Bourne End to Maidenhead with two sets in the morning peak, and then just one thereafter.
Indeed - always struck me as an odd way to run a railway. I'm guessing space is so tight (or someone is so cheap / was expecting Marlow to close too) that they came up with that track layout