Plumbing – the purpose of the valve between these two pipes

You could have a frozen pipe (actually, it would take two in this case to completely isolate the upstairs bleeders.) You might not. Don't know the layout of your pipes...

Circulation pump does not need to be running for bleeding. System pressure alone should suffice for bleeding. "Hissing" when you bled before sounds like trapped air. When you start to get water mixed in, close the bleeder, wait a few minutes and re-open. You should eventually get just water, no hiss, no bubbles.

I'm going to suspect that you have low pressure in the system, so air is not being forced out the top bleeders by water pressure in the system. You should have a pressure gauge (often on the same face as a temperature gauge) on the boiler - what does that read? Normally it should be 20-30 PSI. You may need to add water.

If you can't sort it out yourself, call for professional help sooner rather than later, as letting the system freeze can result in considerable expense.

Honeywell does not provide a ton of info, but from the installation manual it would appear that evoHome is assuming each room has its own feed and return pipe, and in addition has its own local valve. You're unlikely to find such a system in the USA -- and in fact I'm kind of amazed that UK homes commonly have such a layout. USA systems typically have one to 3 or 4 zones in the house, and all zone valves are near the boiler. A given zone will heat a few rooms with a single pipe loop, so you can't control on a by-room basis.

My impression is that this system is much more useful in a multi-office business space, as it allows centralized control of the existing valves in each office space.