You can only play one stadium card per turn, and only if there isn't already one in play with the same name. Also, only one stadium may be in play at a time.
Yes, mega blastoise EX must evolve from the basic blastoise EX. It cannot evolve from the older blastoise ex. Even though it's just uppercase vs lowercase letters, it's enough to change the pokémon's name and make it a different 'mon
You always have a hand. Sometimes, it's a hand of zero cards. As for playing N or colress with a zero card hand, you shuffle the deck and draw x cards.
What is pokémon gb2?
And as DNA stated, new status conditions override previous ones, so the 40 between turns poison gets replaced by a 10 between turns poison.