All the prices I quoted were found on Trip.com.
I have found that approx 75% of the time I can select seats at time of purchase with them, if not I can
do so with the airline afterwards.

You will probably not get cheaper fares booking direct with the airlines - I have consistently found Trip.com
to be equal to or lower than the fares on each airlines site.

I do not want to be a shill for them, but in my experience Trip.com is the best way for a foreigner to book
travel to and within China, and, increasingly, outside of China as well.

I also don't like shilling for them because their buying spree was among the most anti-competitive I've witnessed in any industry (e.g. snatching up the aggregators was the icing on the cake), but it's difficult to dispute their utility, especially for people who don't speak/read Chinese. Here's the thread I started about them, in case anyone hasn't seen it:

The Ctrip/Trip.com GBU thread

In any event, I'd like to reiterate that buying tickets from Chinese airline websites or apps is usually a fool's errand, with the possible exception of 9C.

Edit

OK thanks guys.

So I will buy my train fare as soon as its available on Trip for Shanghai to Beijing as soon as its available. As there will be two of us this time, will I be able to chose seats to be together? I booked this journey last year and I'm sure the only option was window or aisle. We obviously want two seats together.

For Beijing to Xi'an, I will wait until 2 weeks before and make a decision on Train or plane. Maybe even treat ourselves with Business on the train! Does anyone know which trains have the kiosk style cabins? (I've a feeling that may be on Shanghai to Beijing only?)

Xi'an to Shenzhen is all done now, so lessen learned. (I did have a quick search before posting, but alas obviously too quick!)

In regards to the train, I was able to select the seats together for my father and I last autumn directly on the 12306 appli installed on my mobile, although I bought directly through the appli in this instance.

I'm fairly sure I was unable to use 12306 when I tried last year. Doesn't it need a Chinese number? I will take another look.

OK.... I added a journey for 5 March and it accepted me when I added myself as a passenger.. next step was payment - which which I know a purchase wouldn't be made (I think), I don't want to risk getting a strike against my name. (I run into that with grab in KL and get put on the naughty step for 24 hours for cancelling a fare twice for being too long a wait! Luckily, I had already booked the airport run for the next morning! Its stuff like this that make me a little cagey.

Is it possible to see which carriage set configuration is being used? I think we'd probably take the 09.55 from Beijing to Xi'an G55 if its using the CR400BF-Z giving the kiosk style business seating. I'm sure there is a way of seeing this info, but I couldn't see it in the 12306 app.

Yes, the 12306 needs a chinese telephone number (incl. Macau, TW, HK). Which I have, of course.

I would assume the lie flat business class seating would be priced accordingly, otherwise it would be labelled as first class.

Can you guys try to keep the train discussion in one of the train threads, like this one?
Booking train tickets in China?