Thinkpads are extremely well documented. For how to repair/replace parts, you need the HMM. Just Google for “Thinkpad t14 Gen 1 HMM” and you should find the official PDF on their site. That will tell you, step by step, how to replace the keyboard.

As for the part itself, you can again check Lenovo’s site for all compatible parts (FRUs) and find the item number and details. While I wouldn’t recommend buying directly from them due to cost, this should give you the information needed to find it elsewhere. eBay has tons of Thinkpads being sold for parts, and many of these will be parted out. You should have no issues finding what you’re looking for.

They all have to work (at least to an extent) using only x1. It’s part of the PCIe spec.

Missing pins are actually extremely common. If your board has a slot that’s x16 (electrically x8), which is very common for a second video card, take a closer look. Half the pins in the slot aren’t connected. It has the full slot to make you feel better about it, and it provides some mounting stability, but it’s electrically the same as an x8 that’s open.

USB the protocol, or just uses a USB cable? If it’s not using the protocol, the cables are a cheap way of getting cables of a certain spec.

Most instances defederated from them. I’m kinda surprised db0 isn’t one of them.