MOAR SUGGESTIONS:
Blighted Agent
Chained Throatseeker
Viral Drake
Contagion Clasp

for land, I'd suggest:
8 swamp
11 island
4 drowned catacombs

Card suggestions:
Ichorclaw Myr
Distortion Strike
Tainted Strike
Bladed Pinions
Into the Roil
Livewire Lash
Virulent Wound
Plague Stinger

Deck suggestions:
The Blight Mamba is great in a green infect deck but it's basically pointless to try and put 1 Blight Mamba and 1 Forest in the way the deck is currently. That just delutes the deck. (also, 60 cards is your goal, having like 61 won't kill you but the most effective deck is going to run as few cards as possible to increase odds tha tyou get exactly the cards you want)
- Peirce strider is fairly pointless in this deck because getting opponent to 0 hp isn't really a win condition this deck can achieve (since all the dmg is infect counters)
Regarding the crazy number of different counterspells this deck currently has, although it's sort of a matter of personal preference, my go-to counterspells for different jobs are Mana Leak, Dispel, Turn Aside, Stoic Rebuttal, and Cancel.
-Stoic Rebuttal if you can pull off the metalcraft easily is awesome
-Dispel is situational, tends to be good at protecting you and your stuff from your oppenent's attempts to foil any combos you may be trying to do on your turn.
-Turn Aside is great at protecting your creatures if you're having trouble against decks with lots of creature control or decks that have ways of removing that 5 mana enchant this deck is running.
-Mana Leak is a good all around counterspell. Often helps protect a slower deck like this during early game.
-Cancel is just the ultimate "you can't do that" card. The mana cost is pretty high. Leaving 3 mana untapped (unused) on your turn so you can cancel something on your oppenent's next turn slows you down a lot. I avoid running this card as long as I can get the job done some other way but certainly not useless.