GO FOR IT, NEWBIE!!!

...There is no reason to believe that it is impossible to use the Portal Scroll while simultaneously fulfilling several conditions (Guild Hall upgrade, Pandora, taking the Book of Knowledge).

I don't think I stated anything like that. If the corresponding Guild Hall level is built in any of the controlled castles, then the scroll can be used (see my point 1).

My point 3 describes a situation where the corresponding level Guild Hall is available in the starting castle (for example, Guild Hall 4 in the castle for the Portal), but is not built in any of the controlled castles.
In this situation, according to the rules, points 5.2 "a" and "b" do not apply, i.e., neither taking Pandora nor taking the Book of Knowledge gives the right to use the scroll.
Thus, for a player starting with a castle (and all other castles, except for the Swamp and Barbarian castles), there is only one way to use the Portal Scroll - building Guild Hall 4.
For a player starting with the Swamp (or Barbarian castle), there are 5 options:
1) building Guild Hall 4;
2) Pandora with Portal;
3) Book of Knowledge;
4) building 2 Guild Hall 3s, provided that there are no castles under control other than Swamp and Barbarian castles;
5) a Mage with Portal.