I have dual monitors, and pressing the Windows key allows me to move the mouse off the WMC when it is maximized to the second screen - however, my dual screens are identical 22" WS monitors, each running off an independent graphics card (pair of GTX 260s) so mine flickers for up to 2 seconds and then all is well.
That is how I can watch Jeopardy, The Mentalist Numbers, etc. while being on here / playing a game / etc.
I suppose it has to do with your HDMI connection, assuming that your TV supports HDCP - I think it thinks that you are trying to do something 'illegal' like streaming it or recording it or whatever.... If your setup does not do HDCP, then it may be the fact that the All in Wonder cards are iffy to begin with....but you might check your CCC to see if there is an option along the lines of Always enable HDMI port". Sorry, I don't have any ATI cards anymore, and have never owned an AIW so I can't help much more with that....
As for the second option, if you mean simultaneously that is not possible with your AIW (nor, in fact, with most tuners. If you mean that when you run WMC then sound goes through HDMI and when you are not running WMC then sound goes through your speakers, there might be a possibility - but if there is it will directly negate what I said previously, in that you'll need to change a setting (if it exists) for your AIW in CCC to 'Not always send a signal' on that HDMI port, and then see if your sound card shows two separate devices for sound - HDMI and Regular sound - and if so, if it disables the disconnected devices (meaning the HDMI when it is not on) then you can set the HDMI to default, and when the TV turns off (and thus the HDMI gets disconnected) your sound should revert to your previous default - the speakers.
I have a set of Logitech headphones for Notebooks with a USB adapter - obviously when I don't have the USB adapter plugged in the sound app does not show it, hence it disables the USB device, so I used this similar approach to set my speakers as default, then plugged in the USB adapter, and then set it to default.
now, if the USB is not plugged in, the speaker out (feeding my 210W Dolby Receiver with surround and center and main) is my main output. If I plug the USB in, it immediately shifts over to being the primary (default) device, and all sounds are routed to the USB adapter.