Here is the philosophy ( at least as I understood it.) To have a chance against Barca we needed to control possession and at the same time limit Barca's build up.
Now , controlling possession required our best passers on the field, and limiting their build up also required our best pressers on the field.
Those were the keys to the game in Pep's mind. Knowing the above, who of our front 6 is weakest on the press and and who were the weakest at keeping possession.
The answer to the above questions was why Aguero was on the bench. It isn't particularly complexed.
You're somewhat on the right track. But off course in other points.
Barca is a better team - better players. And they play a similar system.
Still, pressing Barca whenever possible to gain possession was absolutely part of Pep's plans. At best, I think Pep hoped for 50-50 possession.
==========
Attackers we had available and their ability to press::
Silva, very weak. Also tires as the game goes on.
KDB, not great. And seems to tire even more than Silva.
Kun - actually pretty good when he puts a shift in.
Gundugan - no problem, but might not last a full 90 if asked to press contently.
Sterling - OK. Great stamina and would at least shadow press when necessary.
Nolito - probably more than satisfactory. Doesn't seem to tire.
'Nacho - also probably OK. Good stamina.
=======
So, of the above to start, pressing full-time as very important and yet ability to score when in possession also important, I'd have started, in order of preference:
Gundugan
Nolito
Kun
Sterling
KDB
'Nacho
Silva
Kun is far up that list. That he didn't actually start either means that:
1) my assessment of Kun's pressing ability is off; or
2) Pep was sending a message - Kun needs to follow instructions, pressing whenever the ball turns over, staying up front, and moving to create passing opportunities and to move defenders out of possession.
In my book it's 2 (yes there are other reasons why Kun might not have started... possible injury concerns which we're not aware of.. etc. Nonetheless, several posters have pointed to a possible dissatisfaction with Kun's recent performances on the part of Pep - not pressing and drifting back to get the ball - that Kun then doesn't start in our biggest game of the season suggests that Pep is indeed not satisfied with Kun's performances of late.