A bit of an update.
We flew into Chongqing today... and a bit of a mixed bag overall.
First of all, I have a feeling that WeChat Pay and Alipay with a foreign card will not work when paying for some public transportation. I say, a feeling, because more than likely I've missed something or missed doing something.
I tried to use it to pay for the Cityflyer bus (the A series services between HK and HKIA), and the bus driver said using my "pay vendor" QR code will not work. I have to use "transfer" instead, but this is also wrong because transfer is for sending money to another account holder (e.g. via phone number), not necessarily a merchant. On WeChat I found there is a "mini app" called Transportation. Even though I verified my WeChat account via passport and so on, when I tried to use the Transportation mini-app, it would not accept my foreign debit card (only HK or Mainland China bank-based accounts). In the end, I only had HKD 50 on me (most drivers won't have any change), so the driver accepted that and forgave the other HKD 20 that we owed.
A similar kind of thing happened when we arrived in Chongqing and were purchasing tickets for the metro. The machines only accept cash or QR code payment. When I tried to use Alipay or WeChat Pay, they present a barcode to scan, which I used the Scan function on both apps to do so. It then allowed me to confirm the amount to pay, but when I did this, both apps said that it would not accept foreign cards.
In the end, with my broken Mandarin and a bit of Translate in between, we only managed to buy metro tickets by convincing someone to pay for our tickets (only CNY 10 altogether) using their WeChat/Alipay, then we gave them a CNY 100 bill and they WeChat transferred back CNY 90 to me. So now I have a WeChat Balance of CNY 90. Not many people at all seem to be able to help in breaking a large bill (probably no surprise given the ubiquity of Alipay and WeChat Pay).
Tip - if you are withdrawing yuan or exchanging yuan (cash bills), make a small purchase before you leave the airport to break a CNY 100 into some smaller bills (to be honest, probably a good idea with any currency). Ticket machines may not accept CNY 100 bills (the Chongqing Metro ones don't). Also try to ensure that CNY 100 bills are some of the newest, because some places will not accept older CNY 100 bills as they can't confidently accept that they may not be counterfeits (it's not that they believe they are counterfeits, but they can't be assured it isn't). Most reputable forex places outside of China and I would assume ATMs would nearly always dispense current-use, genuine bills.
Translate is a bit funny. While it probably works relatively well, it took three attempts at speech-to-text for a waitress at our hotspot restaurant to tell me that the condiments were available to take over there. The camera function (while not live all the time) was relatively good at translating the menu, however. I was thinking of paying the hotpot meal using Alipay or WeChat, but before I could contemplate and agonise over whether they could scan my barcode rather than I scan theirs with the possible result the same as when we were stuck buying Chongqing metro tickets previously, Dad said just pay in cash. That's when we realised that some places were hesitant to take older CNY 100 bills rather than newer ones.
I'm still being admonished for my lack of adept Mandarin.