Hi,
Where you say Canada Water, do you mean Surrey Quays?
1) Yes and no. Canada Water definitely works the same as Surrey Quays. I think Whitechapel may also do.
2) Yes there are pink readers at Whitechapel. You’ll pass them when changing between Elizabeth Line and Overground.
3) You’ll be charged for 3 separate journeys. Slade Green to Elmers End, a tram and East Croydon to Gatwick Airport.