So basically:
1st turn, 7 hp healed, +3 poison.
2nd turn, 7-3 hp healed = 4 healed, + 3 poison
3rd turn, 7-6 hp healed = 1 healed, + 3 poison
4th turn, 7-9 hp healed = -2 healed = 2 damage + 3 poison.
takes 4 turns for poison damage to overcome healing.
5th turn, 7-12 healed = -5 healed = 5 damage + 3 poison
6th turn, 7- 15 = -8 healed = 8 damage + 3 poison.
7th turn, 7-18 = -11 healed = 11 damage + 3 poison.
One widow + death stalker + Arsenic = 6 poison counters each turn
with that combo, 2nd turn poison is overcome .
overall an intersting card mechanic, i like it very much 
Actually not....
By turn 3, it heals a total of 7+4+1=12 HP
so
turn 4: 12-2 =10
turn 5: 10-5 =5
turn 6: 5-8 = -3
Basically if you add the effects together, the total damage will only overcome the total healing after 6 turns. (assuming the healing was useful and not wasted)
The unupped version is therefore even more UP. IMO it would take too long for it to be used effectively. (Again assuming the healing is useful)
Maybe lower the healing to 3 and 5 ?
Umm, i think you misunderstood...
opponent has 100hp
1st, widow (upped version) does -7 damage (heals 7 so plus nothing because of full health) and puts 3 poison counters.
end turn, opponent receives 3 poison damage. now has 97.
2nd, next turn: 7 hp heals, opponent has 100hp again , and then adds 3 poison counters. total poison now 6.
end turn opponent receives 6 damage, now has 94.
3rd, next turn: 7 hp heal, opponent has 100hp again, adds 3 poison counters. total poison now is 9.
end turn, opponent receives 9 damage, now has 91hp.
4th, next turn: 7hp heal, look! opponent has 98hp, not 100 this time, whoa
, adds 3 poison counters, total poison = 12.
I count... 4 TURNS! not 6...
