Hi there, I brought my third son back to China when he was 7 weeks old last summer and have been living here since then. To address your questions in order, I carried 4 tins of Aptamil in my suitcase and was not stopped or questioned about it. But to lessen the chances of that one official making life difficult for you, I suggest you pack what formula you are bringing in your suitcase and go through Customs separately from your wife ie with your suitcase on one trolley, which you push, while she pushes her suitcase on a separate trolley. Then if she is stopped, you keep on walking and wait for her on the other side of Customs.
If worst case happens and you end up without formula, you should be able to find Wyeth S-26 Gold in supermarkets, I know I have seen it in Carrefour and Auchan here in Chengdu. There are also other brands that are made by international companies using foreign milk, your wife can look on the tins and see where the formula was manufactured. Although it is not possible to guarantee 100% that they have not been tampered with, I think the big stores have a lot to lose if they are found to be carrying counterfeit goods so will work hard to ensure that doesn't happen.
That said, we have been importing formula from the UK through britishcornerstore (which you mentioned) to give my son. I imported formula for two reasons - I can afford to, so given a choice between formula purchased locally (which probably travels all over China and sits in a company warehouse for a while before it reaches my local store, thus increasing the window for tampering) and something shipped directly to me, I want the latter. Also, in terms of cost it doesn't work out any more expensive to import myself, as the imported formulas in Chinese supermarkets cost around rmb200 per tin (way more than in the UK) which is in fact slightly more than I end up paying britishcornerstore.
With regard britishcornerstore, I have been very happy with the service I have received. I have ordered formula from them twice so far and both times my order has arrived within a week. The second order was slightly slower as it seemed to have arrived in Chengdu but had not been delivered to our compound so I emailed BCS my address in Chinese, they passed this onto the delivery company and the parcel arrived the next day.
Good luck with your trip, the only other baby essentials that I bring from home are nappybags (the very thin, perfumed bags that you use to dispose of smelly nappies), bottles and dummies/pacifiers as I think the safety controls are better in the UK and I wouldn't want to worry about these plastics deteriorating (and leaching chemicals) while being used by my son. We bought a microwave steriliser for bottles here for around rmb280, a bit more expensive than in the UK. And for nappies, we use pampers and they are fine. You can buy these all over the place ie up in the mountains of nw sichuan where my in-laws are from, but of course the smaller the shop the more expensive the nappies so we buy massive bags from carrefour or auchan.
oh, one more thing, bring your infant car seat with you as there is no charge to bring this on the plane (even internal flights) and these seem to be totally unheard of here, even in big cities like Chengdu, we get people coming up to us all the time asking what it is!