@5. And what is the solution to the finger print reader not working? Could you please just link to it?
Update, it's mentioned in the respective comment: https://de.ifixit.com/Anleitung/Google+P...

Applying the E6000 was very cumbersome, as the cable was quite springy and lifted up the sensors. We needed to weigh the sensors down for some hours to really have them stick.

Any hints on re-applying the tape? It didn't stick anymore properly.

Thank you for that hint! A carefully applied Exacto knife worked much better than the original recommendation, for all of the sensors!

Yes, the diffuser was a black film layer with a white milky circle.

We broke one of the sensors off from the cable, and ended up just having one working ambient light sensor.

FaceID however continued to work with that one sensor in the end.

When inserting the new battery, make sure you position it correctly. This is not easy. If the new battery falls in place too close to the logic board it might not fit back in, or, if it is too far apart, the connection between the two batteries will be under tension.

You won't really have an option to correct this, because the glue will stick immediately, and if you try nonetheless, you might damage the new batteries.

Too bad the guide doesn't give hints for putting the device back together.

Most of the pull tabs were pairwise connected in a loop, and removing them went okay.
However, our Wi-Fi version had this additional long adhesive on the outside as well, on the side that has the battery contacts (lower half in the step 73 picture, about a centimeter in width, and h

That made the battery very hard to remove. As a result of the mechanical stress, that side of the battery swelled up some days after. Luckily nothing was damaged and nobody was hurt.

Also make sure to mark the edges of the battery, such that you can glue in the new battery at the right location.