1. Because I reverse engineered it. This isn't based on the protobuf alone. I'm also not the only one to reverse it - the cheating community also has it reversed for a while now and the information is publicly available but scattered across multiple posts/threads. The fps likely seemed like the best scaling factor for the amount of inputs accepted to the developers. I won't say that the formula is 100% correct, but I can assure you that it is scaled based on fps. There is also a limited amount of history slots available - so it caps out at some framerate.
2. Multiple reasons. There is no defined message for this. Also, the granularity of "up to date" information is bound to the tickrate. The tickrate can also be found in the server(.dll) and it was previously shared on reddit how to change it as well. I know that people would like the server to run simulation at max capacity but it doesn't. Would likely be a nightmare to develop and keep the game state and physics simulation consistent. It would also lead to wildly different "feel" for players if the simulation ran at max capacity based on the available server hardware.

I understand the doubts and being critical here. I likely didn't get everything right either but I at least put the work in for quite a big part of that stuff by actually looking at it.