Great guide so far!
Just thought I would add that I believe the achievement ''Fully Inked'' is also missable. The reason being that some skills have criteria to make them unlockable. One of them has the criteria to take two outposts without being detected. To get unheard you only need one, so would in fact need another one to get all skills. Would be a pain to have to start a new game to re-earn ALL skills if you missed it.
Also off-topic but iBuzz7S's comment is rather rude =/ Especially from a mod. To say part of a guide ''looks like shit'' when it has only just begun? And due to an error with the site note the TC =/