Can we move most of the code to a (x64) page table library instead
of
cut+pasting like this, please?

As I explained in https://edk2.groups.io/g/devel/message/85582, there
has already been a same feature requirement.
https://bugzilla.tianocore.org/show_bug.cgi?id=847 . (Create page
table library to abstract all page table manipulations) I discussed
with the feature owner (Ni, Ray). He said this feature is in his plan
of 2022. He suggest I can wait for the library.

Ok. Can you add a comment to the source code saying this is temporary until
the lib is there, and a link to the bug?