You will find the code at the bottom of the product page, in the "20% off voucher" category.

Photo screws are "standardised" and the quick-release plate comes with both screws (1/4" and 3/8").

They are both RC2, so officially supposed to work.

According to the product photo of the Zhiyun gimbal, you can probably screw the camera directly, so it will have a screw with 1/4" thread. The plate mount has a 3/8" thread for screwing it onto a tripod. So purely in terms of screwing, it should actually work with an adapter from 1/4" to 3/8" (e.g. Manfrotto MN148 (adapter)). However, the whole structure is then 20+ mm higher and probably shifts the centre of gravity, how well this works with the gimbal I can unfortunately not say.

Yes, this is possible. However, the Befree tripod can no longer be folded upwards, so it takes up more space and no longer fits in the bag. The Peak Design PROplate is compatible with the Manfrotto BeFree, simply screw on the two extensions on the side.

There is still one in stock. I ordered and received it this week. By the way, the interchangeable equipment here is complete (plate and slide).