I'll try to keep this as brief (and non-confusing) as possible.

Player A proposes an attack, Player B passes/accepts/whatever so it's legal and the attacker exhausts.

Now, if the attacker was KO'd due to a card effect at this point, would the attack still go through and (potentially) stun the defender?

if there is no attacker or defender on attack conclusion then there is no longer an attack

There is still an attack.

The attack will conclude with no effect. But even after the KO, as long as you have not left the attack substep, you can still play effects that target a defender (who is still around), or other cards that require an attack to be occuring.

this is very important to remeber when team attacking.

One of my latest decks included the use of insignificant threat. I love to use that during a team attack. Exhuast a character, stun an attack, the attack conlcudes and I stun the other attack and do not get stunned myself.

The important thing to remember is when the attack concludes (and they ALWAYS do) you compare all the remeainging attackers to the defender and what not. If there are zero attacks the attack does nothing.

I love it when I have Grodd on the board and my opponent team attacks someone and I can do Grodd tricks...

How about during a team attk, a defender was stunned by a card effect and was recovered using another card effect. Is the defender still being team attk or does the team attk ended immediately once the defender is stunned?

Neither. When the defender was stunned, he was also removed from the attack. Recovering him does not make him the defender again.

But the attack did not end immediately on his being stunned. For an attack to move to conclusion, each player must pass on an empty chain.