How do I test the ad-supported free tier of Xbox cloud gaming on PC?

Switch Pro is not a supported controller. There is no information when or if more will be added

https://support.xbox.com/en-US/help/games-apps/cloud-gaming/cloud-gaming-tested-controllers

You paid for a subsidized piece of hardware, and part of what pays for that is advertisements. A console is not a PC. If you want an experience where you’re 100% in control, then that’s where you should do your gaming.

Ad deals are signed with certain guarantees of exposure. While it might be possible in the future for them to offer some kind of Kindle style buyout, or ad free tier of gold, they have not decided to do so at this time

As the cloud is currently exclusive the game pass ultimate outside of Fortnite, they already do

I have no way of knowing and cannot speak for the future, but you already require a game pass ultimate to play any game except Fortnite. Given the expense of maintaining the servers, a game would likely have to be able to generate that kind of revenue that fortnight does in order to be added to a free tier, And if that revenue where to dry up, situations can change. That's the best answer I can give you

Currently the versions running on cloud are the Xbox system versions, and not the PC. Currently the protocols and algorithms that allow cloud gaming to function are only tuned for the Xbox controller. Well additional control schemes may be added in the future, currently only the controller is supported, as the system is still in beta

I'm sorry Fortnite is a grand exception and it is very unlikely that any other game will ever be added as an open free cloud title in the foreseeable future.

free to play games already or exempted from the requirement for a game pass subscription if you have Xbox hardware

Supporting cloud gaming is extremely expensive, which is why a subscription is required

if you purchase an Xbox, you'll have many free to play titles available to you as well as games to purchase