I'm in the same boat as you. I have a double-DIN space and can't decide where to mount everything. I would like to find another dash panel to hack up (while leaving my original dash panel intact). that way I may be able to fit both a double-DIN LCD flush mounted with a single-DIN HU underneath it (if space permits). that could be another option for you to consider. like you, I don't want the PC to completely replace the HU, I just want it to be an addition to it.
I'm probably going to end up going with a single-DIN HU in the lower slot and a motorized LCD in the upper slot (like your first idea), unless somebody convinces me otherwise.
Your second option would be ok if you don't have a theft problem where you live/work and don't mind the loss of visiblility; then mounting a screen above your vents will be the cheapest and simplest option for you. also, I don't know about the laws where you live, but in the US you would get pulled over in a hearbeat with a huge 10" screen blocking your windshield view like that.
good luck, BTW nice car