1. Before you start with Multi-Tool

Both of my belt's were way overtight as well. I also had to drop the nozzel seals alot.

This is literally the first run.  Saying "Make sure you are running Firmware 5.1.2 or newer" when i haven't even booted the machine yet seems odd.  Perhaps a step to download the right firmware onto a usb stick?  how to check?

Did I miss something? The thin plastic band that runs from the back of the printer to the Nextruder head - between the bowden tube and the cable - the part that is secured to the Nextruder with 2 screws - looks like it should fit inside the verticle part with a "U" cross section. Should it be secured to it or snap in?

Before continuing, in addition to checking the upper rear stepper motor grub screws (two per motor) on both sides as mkoistinen says I recommend you also check the belt tension using the phone app as shown at https://help.prusa3d.com/article/adjusting-belt-tension-xl_401793.  My right belt was so over-tight from the factory (4+ turns of the screw) that when I adjusted it after the following steps, I ended up having to recalibrate all of my docks and tools because the tool changer would no longer pick up any of the tools and the offsets were no longer even in the ballpark. As a side note, if you don't have a decent quality low-profile 5.5mm offset wrench, you might want to get one.  The multi-tool has a pretty loose tolerance on the 5.5mm notch and it quickly deforms, making it worse.  You're likely to strip out both the wrench and the hex heads if you have to do the following steps repeatedly.

I strongly suggest before anyone continues from here to check your CoreXY assembly for loose worm-gears (I had some) and correctly tightened belts (my were over-tightened, and caused really bad layer-shifting once I started printing things). Prusa probably had everything right to begin with before they sent the pre-assembled CoreXY assembly, but for whatever reason, I needed to tune it up. You may need to as well.
Side-note, I'm kinda surprised there's not at least a step to check these things in this manual... perhaps I missed it somewhere?