No probs munitalP..... as you mentioned earlier, there is a lot of conflicting information out there.
Although your wife is spoiled for choice, I reckon you should not miss Xian. I was unimpressed with PVG. The entombed warriors are incredible and Xian is a very nice city to boot. One of the best places to eat in all China. It'd be very easy to tack on a few days there after Tibet (seeing as though you've already done the Ice Festival in Haerbin).
Anyways, your questions:
Altitude: I had hallucinations and mild nausea brought on by extreme shortness of breath only when at Everest Base Camp. We were only there for around 14 hours. It only happened during the night and I didn't sleep a wink. By morning it had sorted itself out. No-one I spoke to had any similar problems. As far as general altitude is concerned, the wife and I didn't have any serious issues and neither did any of those we spoke to. We trained it up from Chengdu (the train ride is superb and assists with acclimatisation). If you choose to fly, just factor in a day of doing very little after arrival. We did notice being puffed from simply walking up steps occasionally.
Guide: It's difficult to say the cost of the guide because it was included in the overall tour price. Wife and I did a private tour, meaning that we were our own "group" and we paid for a guide, a Land Cruiser with driver, (and their costs), all accommodation, train to Lhasa, pick-up and drop-off, some entrance fees, park fees, permits, etc. The price you'll pay depends on your desired level of comfort, where/how far you go, and how many days you stay there. We paid around USD$1800 for our 12 day trip.
Our guide was a Tibetan girl (we insisted on a Tibetan guide) and it is highly unlikely she had any links to any government. The only rule we had to follow was not photographing the military (there are a lot of armed soldiers in Lhasa - completely unnecessary and somewhat upsetting to see around the Barkhor IMO). However, we chose the itinerary and locations we wanted to visit (with some input from the company regarding distances and durations). I understand there's only a few areas that are off-limits and these are way way off the beaten track.
Check out Spinn Cafe on the internet. They were very helpful. We would have toured with them but they couldn't organise our transport. We really didn't want to fly in and train tickets can be very hard to get at certain times as they only can be bought 10 days prior to departure. For us, who we toured with came down to who could get us train tickets for the day we wanted.
