MTG Salvation

My opponent attacks me. I declare a blocker. Then I play grip tide to return his attacker to his hand. does my "blocker" assign damage to my opponent or it just fizzles out?

is an activated ability a spell? i.e. my opponent used Bow of Nylea (which was already on the board) to send 2 damage to my flying creature and kill it. i played "negate" (counter target non creature spell) and said it countered the 2 damage. was i allowed to do this?

Thanks everybody, your answers were very helpful!

I am new to tournaments…I have found that at the local store, it is commonplace for when I say "end of turn" my opponent says that "during the end of YOUR turn I…" and then proceeds to tap for mana and cast a spell. Then of course it's his turn and he untaps his land and plays his turn. So basically he is getting a free spell cast in my opinion. Is this allowed? I would think that when I declare my turn to be over, it's over. Or do the rules of magic not allow me to decide that my turn is over?

Cheers
Elvisrobotico

Can a Planeswalker block? Specifically, Xenagos, the Reveler? The 2014 rulebook says yes, but I read what I thought was a WoC blog and the guy said no.
Cheers,
elvisrobotico

Hi, I have an older card, Tranquil Domain, Arabian Nights I think. Text is: INSTANT: "destroy all global enchantments". Need to know if:
a) would this would destroy "Enchant Creatures"
b) would this destroy Enchantment -Aura Enchant creature
c) would this destroy Enchantment Creatures

I am not sure because the card says "global enchantments" and I do not know if this affects the above.

Cheers
elvisrobotoco