I think I can help with a few of the questions you raised.
I also have a CV1 Rift and have run into the same port problems. For the HDMI, I am using a passive DisplayPort to HDMI adapter without any problems. Mine is Insignia. Short black cable with a plug on either end.
Regarding USB, for mine I just plug into whichever ports are available, including USB 2.0. Admittedly that is purely anecdotal, and I am running 2 instead of 3 sensors. In theory, even a usb 3.0 hub on a 3.0 port has more bandwidth than multiple 2.0 ports, but I don't know how polling rates or other factors would affect that. So you may be able to use your 2.0 ports without issue, it's worth a shot at least.
As to the GPU, you should be fine with the 980ti. Maybe a repaste is in order, given its age.
I used my Rift for a long time on a 1080 and 4790k. So, comparable GPU and less CPU than you have now. I definitely had to turn down graphics settings in-game to get a smooth framerate, and also needed to let the alternate frame interpolation do its work. I don't remember if that setting was in the Oculus settings or nVidia control panel.
Sorry, no input on the space issue, I have always used mine seated.