...these games use 99% of the CPU...
You're doing it wrong (TM). I am working on a game right now, 2 months so far, which has animations and other eye candy and uses no where near 100% CPU on a 4 year old core2duo laptop. It looks like the original Legend of Zelda on the NES or FF 1-6. http://www.cindervale.com/
... and it runs on everything back to IE 6 AND BEYOND. Fully supported browsers include IE 6 (2001), IE 7, IE 8, Firefox 1.0, Firefox 2.0, Firefox 3.0, Firefox 3.5, Safari 3.1, Safari 4, Opera 9/10, Chrome / Chromium, and iPhone.
What people don't realize is that you don't need the canvas element. If you use the canvas element, you are defeating the purpose of a web game since the web is all about accessibility. In a few years, yes, use it heavily! By using the canvas, you create an artificial barrier to entry for your players by saying "your must be on the bleeding edge to play."
There's no decent way to manipulate sounds
100% agreed! Hell, you can't even use MIDIs anymore!
There's no way to switch to full screen or to capture every key stroke/mouse movement.
I can't think of a single key on normal keyboard that can't be captured. Shift, alt, control, etc are all capturable. Mouse movement is the same.
As far as full screen, have the user press F11. All browsers I'm aware of use this same binding. Then use a bit of JS to get the desktop resolution and the window dimensions to verify.
It seems that what you are experiencing is game design problems. Try designing games with the limitations in mind rather than trying to design a game then making it fit with the technology.