[API /session v3 no longer works]

The posts here enabled me to achieve what seems to be a workaround for the problem by switching to using V2. I am not sure there is any disadvantage to this.

[Option intraday historical data]

I am not sure there is a simple way. You can do it using the API.

[Option intraday historical data]

You can download it. The problem is the limit of 10,000 datapoints per week is tiny when there are calls and puts for many strikes.

[Can't access demo account]

As of now, the web interface appears to be working!

[Can't access demo account]

When this happened, I thought it was deliberate deactivation of the demo account, but now I see it is probably a site bug. The buttons diverting to the live accounts would be an odd behaviour to implement deliberately, so I suppose that is a good thing! 

Note that trying to login to the demo account rather than using live credentials and then pressing the button for a demo account doesn't work either.