if you cant even reboot the device, there is not much you can do.
seems you have attempted all that can be done. this phone is no different than other htc phones in the ways you can power it up into safe mode, etc.
you can search on the internet for other htc phones and see if there are other tricks you can try. i just think the longer you wait around trying to fix it on your own, you could be wasting time that could be spent on getting it fixed.
i take it you are NOT rooted?
and you do not have insurance through your carrier?
at this point I agree with Rush, let htc get it. you still have the htc warranty, let bell send it in on their coin.