5. Termini without changing ends? None by definition as to change ends you would need to work in a loop, as in Kennington, of which, as far as I am aware, there are no such stations on BR.
Bedford-Bletchley and the Dudding Hill branch aren't electrified, so the only termini north of the Thames Thameslink services could terminate at are St Pancras and Moorgate if there is still public access. A diesel train could terminate at Euston (via Bletchley), Victoria via Dudding Hill. I am sure you could get to any of the BR termini via convoluted routes without reversal.
Non terminal stations within London they could terminate at are Kentish Town, City Thameslink, and if it is in London Radlett. West Hampsted has a crossover but I doubt it is signalled.
London Bridge - via Sydenham, Crystal Palace, West Norwood, Tulse Hill, Peckam Rye - London Bridge.
London Bridge via Norwood Jct, W. Croydon, Sutton, St.Helier, Wimbledon, Streatham, Tulse Hill, Peckham Rye, London Bridge
Victoria - Clapham Jct., Balham, Norbury, Selhurst, W.croydon, Sutton, Wimbledon, Streatham, Tulse Hill, Hearn Hill, Victoria
There are may others on the soutern