I'm in a quite different climate, but Guernsey Cream is a full two weeks earlier to bloom than other type 2 clematis in my garden, even though it is in a shadier spot.

Go for it! After three years, mine finally bloomed this year and it was outstanding! Mine is in full shade too. No tips except patience. Here's the website where you can find out all sorts of things. http://www.clematis.hull.ac.uk/

I'm not sure how much stratification it needs - others may know better than me - but i think seeds need a period of stratification, several weeks or several months. So if C. tangutica needs a longer period of it, just a day or so wouldn't do it. I just checked my WS records and i sowed C. tangutica on March 6 and it germinated in May. Were these seeds from the box? I might have put them in! Hope they germinate for you. I tried some again this year, just to do it, and they haven't sprouted yet.

I'm not sure where you got the idea that pinching would encourage new vining stems. It doesn't. It might encourage bushiness but not more stems (which is what I think you mean by "branches"). The additional stems/branches will originate only from the root crown. You encourage these by A) planting the root crown deeply (4-6") in a generously sized and well amended planting hole and B) hard-pruning the vine in late winter/early spring for at least 2 growing seasons. Also, fertilizing with alfalfa meal or alfalfa tea will also stimulate additional stem development. Alfalfa contains a natural plant growth hormone that promotes this type of growth. But it is getting a bit late to do too much more fertilizing. And don't expect it to happen very rapidly. If you follow these practices, I wouldn't expect see any changes until next season's growth begins.