So... Is it absolutely impossible to, say, enter tibet from kathmandu, spend about a week there, move on to china, and then tour around china for 6 weeks...
if #10 has two months for travel in China, it's better to go to Tibet from inside the country, not from Kathmandu. Otherwise divide trip in two parts - first go to Tibet and then China, leave the country and return.

Davyd : For some reason I cannot reply to PM's, so please see below.
Hello,
Yes I did book my train ticket through them, and it is expensive, they charge $100 commission as the train ticket for hard sleeper only cost 712RMB and I was charged US$180 to Chengdu. I don't think it is possible to book train ticket yourself in Lhasa, unless you try contacting a Lhasa agent. A couple people on our trip did book the flights themselves on line, but it worked out the same as what they were quoting, I think for train tickets you have to book through Kathmandu agent as they are responsible for making sure you exit Tibet.
Cheers Tina
Just got Chinese visa. It's for 28 days. Leaving tomorrow morning. I'll post how my journey to Lhasa proceeds.
greetings from Lhasa. it's very bright and colourful city which I did not expect.
Right now there are some winter festivities and yesterday at Jokhang there were thousands of pilgrims from all over Tibet. Jokhang temple imho is the one attraction not to be missed in Lhasa, it's just amazing.
After Lhasa all tourists in my group depart from here differently, some take train to Beijing or Shanghai, few fly back oor drive back to Kathmandu.
I am on my own, booked train ticket to Xian, Shaanxi.
So in a word now I can say it's possible to make journey from Nepal to Tibet and onward to China.