Yes you can add park hopping to your ticket on your 2nd day of use. It will cost the same as if you added it on your first day. It is as set cost to add no matter how many days remain on your ticket
Yep. Disney doesn't pro-rate. To do 2 days the way you want to, you could buy one 1-day hopper ticket and one 1-day non-hopper ticket at a total cost of $254. A 2-day park hopper ticket is $225, so even if you only want to hop one of the two days, you are still better off with a 2 day hopper.