It is a bit weird, but once you hit your limit with silver cards, you need to go back to the beginning and battle green cards. Then go back and battle blue cards. It is not linear.
Twice a day is possible, but I don't sleep much.
This is quite annoying yes, I just take any two random cards now as it is too much trouble to check it.
I don't know if anyone would really mind, I don't, I would have bought the ones which are for sale if buying would work.
Viagra is really too rare, I can hardly upgrade any cards.