I travel quite regularly with CZ. I make all my own bookings on their website and select seats without issue. Even when I need to change or add something, it’s been straightforward online or with the call centre in China. Have never been asked to email anything. By the way, I’m unclear whether they are asking you to email your passport number or a photograph of your passport. You can easily confirm when you loaded your passport number at purchase by printing a copy of your order. I may be wrong, but I don’t think it’s possible to complete purchase without loading those details. I’m still not clear why you just don’t go online and select your seat....it’s prety easy to do. Some posters have had difficulties with the website, if that’s the case, then try the app. It’s much more stable and effective than it used to be.