That step is immediately above this one. Check out step 21 to 23.

There's two kinds of thermal paste used originally for different purposes. For use between CPU and heatsink, I recommend something high quality like Arctic MX-4 or better. And the other between heatsink and the shell it could be whatever passable. If you just don't want to stock two variety, use good one for both. MX-4 is quite cost efficient but there are other choices too. 2gram should be about enough.

No they are different.

No. You will need for professional help to replace the whole ribbon cable connector (ZIF connector).

As far as my result goes, it is preferable to choose some less watery, more firm thermal compound on both. As long as it does not run around, it is fine to use the same on both. I have not used Arctic Silver 5 so don’t know how that works. But I have used Noctua NT-H1 and Thermal Grizzly Kryonaut, both working well. I’ve tried something quite watery from Prolimatech which quickly stopped working after like a week.