This is a technical limitation that Android application cannot obtain keyboard event for ALT+TAB. To push a window to foreground, one way is to use the unityTouch sidebar, open the sidebar, and in the "running apps" section, clicking on an item would push the corresponding remote window to foreground.
Perhaps Horizon Client for Chrome from Chrome Webstore could work for this ALT+TAB requirement. It is worse a try if ALT+TAB matters for you.
There is no scaled resolution or customized resolution in Horizon Client for Chrome. Change the resolution/scale in the Chrome OS display settings may help. ChromeOS settings -> Devices -> Display.
Managed public session is the special usage of Chromebook, if the display setting of ChromeOS would be reset, I'm afraid any kind of application data, like previous connected broker address, would also be reset. It seems to be expected behavior for managed public session. I think the way to avoid settings being reset, is to use an account to login the chromebook.
VMware supports setting the broker address etc using JSON settings https://docs.vmware.com/en/VMware-Horizon-Client-for-Chrome/5.4/chrome-client-installation/GUID-DEA8... Also supports setting the resolution, but I would have to have separate JSON settings for each resolution from what I see. https://docs.vmware.com/en/VMware-Horizon-Client-for-Chrome/5.4/chrome-client-installation/GUID-B868...