The answers for nearly all of your questions are in the opening post.
Distant Advantage (Wood Elf racial) not only provides +5 ACC for Whisper of Treason and Flames of Devotion with the arquebus but also gives you bonus deflection against ranged attacks. It even works with Sacred Immolation. It's explained in the build description.
Island Aumaua would be an alternative if you want to spare talent points and still have 3 weapon sets: 2 for arquebuses for FoD and the third for your weapon & shield. You don't use the weapon & shield setup all the time. You want to use the arquebuses for Flames of Devotion shots.
It is mentioned several times that the order is a Shieldbearer (of St. Elcga).
8 CON is enough if you have good deflection. INT influences the duration of Whisper of Treason (which is a key ability of this build), it also expands the area of effect of your zealous aura and the radius of Outworn Buckler's Herald aura. It also expands the duration andarea of effect of Sacred Immolation. It also raises the healing you do with Lay on Hands. RES only raises deflection and will a bit which is not needed in a party setup.
If this should be a main tank then more defense and less offense and more CON could be better. However single main tanks don't work well in PoE as they can't hold too many enemies engaged - and once you play with two or three dedicated front liners you don't need extreme tankyness but be better off with "tanks" that can also contribute to the party's performance in other ways. This build has great supporter qualities (heal, boost party ACC, boost single ally ACC, boost party's deflection, raise single ally's deflection), can charm very tough foes easily (great single target CC - easily charms dragons for example) and also do serious burst damage (can take out enemy squishies easily with a well placed shot).
Athletics is a poor skill compared to survival. Consequences from having low Athletics in scripted interactions are not severe or really bad. You can easily do a playtrough with a party where nobody has even 1 Athletics.
Yes - most abilities and talents get only unlocked at higher levels. The ones I named are not ordered by level. Just pick the ones from the list that are available at lvl up.
Edited by Boeroer, 06 July 2018 - 09:04 PM.