To avoid confusion, STR, DEX, INT are attributes, where as Armor Specialist, Willpower, Leadership, Crafting are abilities.
Just to be clear, are you saying that the game won't allow you to add points via the "+" button to at attributes once it's been raised to 15, but that you can continue to raise it via item bonuses past 15?
Assuming that is correct, if you manually assign points to your INT via the "+" and raise it to 13, and are wearing gear that gives you an additional +3 bonus to INT for a total of 16 INT, can you still add assign another point to INT via the "+" button since it is technically the gear pushing you over the cap?
Yes... once you hit 15 base that's it. You can no longer increase it naturally. Only through feats (kia), items, and presumably spells can you raise it higher.
On lonewolf you also have a surprising number of attribute points.
By level 14 I'd be able to cap out Aero/Hydro/Witcho without the benefit of skill books.