I'd rest easier with a later CA connecting flight. I don't think your only other choice is the PEK connection, which I wouldn't take under any circumstances since you really don't have all that much time there either. A sure loser.

There are CTU-LXA flights on CA most days in the early afternoon between 1-2 pm, sometimes as many as 3 flights. See if you can change to one of those. Much better to know you have to kill time at CTU or in Chengdu city rather than worry about misconnect. Yes, that means you'll lose a few hours in Lhasa.

Failing that, I'd gamble that I'd make the connection and if not, could sweettalk CA into putting me on a later flight. The iffy part is seat availability--if traveling into Lhasa near a Tibetan festival time or in high season between July and September, availability might be dicey.