Mogao Caves tickets: The online website is practically unusable, I'd skip it. At any rate, you can't book advance tickets too far in advance. For a September visit, my advice to you is to contact your Dunhuang hotel and see if they will pick up a ticket for you a few days in advance of your arrival. If they cannot/will not, then just go to the in-town Dunhuang Mogao ticketing office as soon as you get into town, and buy there for the next morning's English tour. I very much doubt they will be sold out.
Itinerary: I completely disagree with poster #1. You have plenty of time to do all this but some of your methodology needs a tweak--there is some car/driver hiring that you'll need to do to make this practical. Some of these bus journeys won't take but a few hours and you'd have time to fit in more. Specific comments:
--Days 1-5 work fine. Nicely set up and paced. On Day 4, you'll need to engage a car/driver. Mati Si and Danxia are in different directions out of Zhangye city, and public bus transport in September isn't going to coordinate timing wise. I did not find that Mati Si needed more than about 3-4 hours onsite (including the 1000 caves location which is about a km away from the main Caves).
--Xining part: Engage a taxi from Xining City to Kunbum, wait, then return. Not all that expensive and it saves you a bunch of time. This excursion will only take part of Day 6. I also think that you should not deal with Qinghai Lake by public bus. Prearrange with a local Xining agent, a 2-day (Days 7-8) visit that includes car/driver round-trip from/to Xining, and accommodation for a night--in September, staying in nomad tents is usually still possible. You may need to take food/water for the trip rather than depend upon local supplies. Not all parts of Qinghai Lake are open to foreigners, and for the accessible areas, the locals will ask for a fee. Qinghai Lake probably won't have too many birds around so the What to Do? is mostly visual, taking photographs and hiking around. So, two days should do it. Try Tibetan Connections out of Xining for advice/set-ups.
--Day 9 If you take my above advice, this will allow you to spend night of Day 8 in Xining. Therefore you can take the morning bus to Kanbula. I haven't been in there so have no accommodation recommendation, but some of the villagers in various places might be able to put you up. (unofficially of course, but should be workable as long as no snoopy police are around). There's some hostel/guesthouse lodging in some of the villages just outside the park, but commuting in daily might get pricey and annoying. Consider yourself an explorer for this segment: you get to write the manual for the rest of us, rather than follow one. :-)) Day 10 as you have it. Day 11: Should be doable though be prepared with enough cash to fund a car/driver to get you to Tongren, should the bus schedule not work out for you.
--Days 12-13 AFAIK there's only one 08:00 bus from Tongren to Xiahe so if you want to spend more time in Tongren, you'll need to move Xiahe to the following day (13). Otherwise, the journey is only 3-ish hours so it's absolutely possible to bus to Xiahe morning of Day 13 then do some walking around and then take the afternoon English language "tour" with one of the Labrang monks. I highly recommend doing this, as you can usually get access to parts that you otherwise couldn't see just roaming on your own. It's not expensive and usually there are only a few English speaking foreigners that show up so not a big mass group tour thing.
--Days 14-17 as you have it. Note that the final segment of the high speed train line east of Lanzhou is supposed to be completed and come online this summer, so in September, you may have the option of covering the Lanzhou-Tianshui journey faster. This would allow time on your Day 16 for a look around Lanzhou before heading on.
--Days 18-19 As you have it. The Chengdu region can soak up any leftover time.
Overall, I think it's a great itinerary.
