Someone telling Player A to table their hand is providing assistance, this should not be allowed.

Someone announcing that the dealer has misdeclared Player B's hand is not assisting Player A, they are correcting a misdeclaration of a hand. Misdeclaring a hand is considered so serious that if Player B had done it the floor would have the option of making them forfeit the pot even if Player A mucked.

"that's not a straight, better table your hand Player A" is wrong.
"that's not a straight dealer" is correct and a player not in the hand has an obligation to point it out if they see it.

Yes, I am suggesting that "showdown" is when all players who have cards have tabled them. And to be perfectly honest, im not crazy about the dealer declaring any hand until this has happened (though i recognize that real world its frequently the way it happens)

The bolded is rather my point. Cards speak, and to my way of thinking player A makes an error in relying on the dealer's declaration. He should have done exactly what the OP did -- looked at the cards and figured it out for himself. If he can't do this, because hes inexperienced or for some other reason, then he can table his hand in the presence of any uncertainty and work it out from there.

What if random player X says "oh thats a straight". Is A now entitled to have someone step in and tell him X is wrong too when deciding to muck his cards.

I think that, like most cases of dealer error, its a crappy situation. But I also think that A has a fool-proof, sure fire way of making sure that kind of crappy situation never negatively effects him -- to table his hand and thus protect his interest in the pot when there is any uncertainty.

This strikes me as a pretty bad rule (and I cant recall ever seeing a room that had it). It just doesnt seem that unreasonable to me to expect a player to look at the cards their opponent has tabled before tossing their own cards into the muck.

It is a very standard rule and is in RRoP (and I dare say a very good rule as it stops all kinds of angleshots). Once all betting action is completed that's it, best hand wins. If dealer says nothing and Player A mucks then tough luck. If dealer has made an error then the dealer error needs to be corrected which is what happened here. Player A mucking due to incorrect information provided by dealer is a very different situation to Player A mucking by himself.

ill take your word for it about RRoP... and you might be right about it being on the books.

that said, its a rule ive never seen enforced, and in the game I play (LO8), misdeclares are probably a lot more common than in NLHE.

that said,

what about player A mucking due to incorrect information provided by another player. What about player A mucking because someone walked by the table and said "ooh thats a straight"

My answer is that its player A's responsibility to look at the board and the tabled cards before he mucks. You don't need a rule to prevent angleshots like this -- you need players to understand that they have to pay the tiniest modicum of attention to hands with which they are involved.
But thats not it. Player A has a decision to make -- the decision to table or muck his hand. You may think this is a trivial decision, but even you acknowledged that he is not entitled to assistance with it, even though all the betting is finished when you wrote:
So there is a decision to be made here, and you concede that it is a decision that A must make on his own.

ANd he makes that decision by looking at the tabled cards and deciding whether he thinks he has a winner. There are all types of things that might lead him astray. The dealer might misread a hand, a player might misread a hand. Maybe, if hes playing O8, he doesnt know that his 23 beats my live ace, or that his live ace plays as a low.

The moment he tables his hand, he is entitled to all the help he can get figuring this out. But until then, at least in my view, its optah

Actually, i take that back. I won't take your word for it.

note the bolded parts.

I might have misread the OP and thought the cards had been tabled. I agree with AEPoker. It is Player A's responsibility to table his hand. If he mucks, that is his own fault for not being able to read a board. I wouldn't say anything until he tables his hand. If at that point there was confusion I think ethically the other players should speak up.

OP - maybe you can clarify. You said:

but you also said:
Had he tabled his hand? If not, the dealer didn't really need to be corrected on anything.

Dealer declared Player B's hand as a straight. This was an error deserved to be corrected by anyone at the table who recognized the mistake, INCLUDING Player B. Doing any more than simply pointing out that "B" does not hold a straight is unnecessary.

Right...I'm not having any luck with reading comprehension ITT atm.

This is correct.
The idea that players not in the hand should not assist in reading tabled hands, or should not correct errors, or should even wait until all hands have been tabled to read anything, is simply wrong.
If you check OP, Player B's hand had been tabled, and misread by dealer, and B failed to correct the misread. Player A's hand had not been tabled. At that point, it is completely correct for anyone at the table to correct the dealer (or to read B's hand, even if dealer hadn't done so).
It would not be correct to tell A to table his hand (OPTAH violation). But waiting for him to table or muck without correcting the dealer's misread would have been wrong, too.
The Showdown begins instantly when all betting is finished, NOT when all hands have been tabled or mucked. It is correct for the dealer or any player to read any hand which has been tabled, as soon as it's tabled, and not to have to wait for the rest of the live hands to show or muck (which decision may depend on the correct reading of another tabled hand).
Here's more complete version of RRoP on Showdown (note emphases here):

"2. Cards speak (cards read for themselves). The dealer assists in reading hands, but players are responsible for holding onto their cards until the winner is declared. Although verbal declarations as to the contents of a hand are not binding, deliberately miscalling a hand with the intent of causing another player to discard a winning hand is unethical and may result in forfeiture of the pot....

3. Any player, dealer, or floorperson who sees an incorrect amount of chips put into the pot, or an error about to be made in awarding a pot, has an ethical obligation to point out the error...."

Yes, players should protect their hands and their action, and pay attention. But the overriding principle at Showdown is that the best tabled hand should win, and that verbal misreads aren't final (cards speak). (But of course, the cards can't really speak until someone reads them..... )

Yes, you are in the minority, ainec.

Here, I'll give you the whole pizza: