The fact that WebForms were deprecated should not prevent this forum trying to assist those of us who have invested many many hours on apps for this target. As Bruce is suggesting in his blog quoted above, there is the option to stay with WebForms. Life will be increasingly difficult but in the meantime you can choose to limp along whilst persuading your end-user community to use IE9.
Your second question first: No I don't think there is a way of getting the user-id.
The first question:
I find that if I don't specify the full URL actual page, you end up with a full screen window:
"http://www.cloudclock.co/skipper" becomes "http://www.cloudclock.co/skipper/default.aspx?clientWidth=1399&clientHeight=991", the url is untouchable, and browser takes up most of the screen.
However if I include "default.aspx" in the request like so "http://www.cloudclock.co/skipper/default.aspx" then the browser stays the same size and the feel is more like a normal browser based application.
I have found when you resize the browser window to smaller than the clientSize you get scrollbars.
hth
Lars