Hi sorry! I meant to respond earlier! Yes all stats capped at 18. If you max a character out, I would retire them!
Double down is on defensive rolls yes.
For the counter, you have to roll 2d20 and add them, so let's say you rolled 10 and 10 for a 20. Well that is less than your Str and dex combined (27) so you would succeed. But if you rolled 2d20 at 15 and 16, for a 31, the counter would fail since it is above the sum of your str and dex stat (27).