[Buzz]

"Online 2-4
Ac2cTsQh in the big blind.

2 callers to MP, who raises. The raise most likely means A2 but I've seen this player raise with good high hands and good 2 way hands. 6 of us play for 2 bets each."

Calling seems right to me. Besides the A2 combo you also have the nut club draw
and some nice high card combos (AQ, AT, QT).

"Flop 7s6s6h
No high for me. I'm playing for 1/2 or 1/4 of the pot. EP bets, same MP raises. 4 call, including me. 10 big bets. Should I have considered letting this hand go?"

Yes.
You need any 8, 5, 4, or 3 without an ace or deuce on either the turn or the river.
4* 4*3/2 + 16*12/2 + 16*25 = 520. (never a guarantee my math is correct)
520/990 = 0.525 is the probability you'll make the nut low.
Odds are thus about 11 to 10 you'll make the nut low, close to even.
But because of the strong possibility of getting quartered you should fold here.
However continuing is only a small mistake, IMHO.

"Turn Jc[7s6s6h]
Checked to MP,who bets. CO raises. All call. Is my call correct? If I've calculated correctly, pot odds are 2.75:1 and I'm drawing at 1.75:1. BUT there is the possibility (probability) that I'm drawing to be quartered."

28 cards on the river are bad for you. 16 cards are good for you.
Therefore, as you have stated, your hand odds are 1.75 to 1.
16/44 = 0.364, the probability of making the nut low, may be more useful (see below).

It's not clear to me exactly how much is already in the pot.
I think you meant there were 10 *small* bets (not big bets) in the pot before the flop.
(10 small bets = 5 big bets)
If so, and if you have four opponents, they are putting in a total of 8 more big bets
on the third betting round, making a total of 13 big bets.
You'll put in 2 big bets on the third betting round to call.
Let's tentatively assume that there will be there will be one bet on the river
and that two of your your opponents will call.
Let's estimate you'll get quartered for low about half the time.
When you make the nut low and win half the pot, you'll win 6 big bets.
When you make the nut low and win a quarter of the pot, you'll win 1.5 big bets.
When you miss on the river you'll fold and will lose 2 big bets.
Assuming you'll get quartered half the time,
0.364*(.5)*6 + 0.364*(.5)*1.5 - 0.636*2 = +0.09

Since the number is positive (though barely so) calling is reasonable, IMHO.
It's close. If we had estimated your chance of getting quartered at more than half,
then the total would have been negative and folding would be in order. As I already
wrote, it's close.

Here's the calculation if you estimate you'll be quartered 60% of the time:
0.364*(.4)*6 + 0.364*(.6)*1.5 - 0.636*2 = -0.07

"River: Js[Jc7s6s6h]. MP wins with spade flush after river checked around. He did have A2 of course."

If you had known for certain MP had A2, then you should have folded after the flop.
However, you weren't certain. I guess the lesson for you is to somehow get to the point
where you are more certain about the cards your opponents hold.

Just my opinion.

Buzz