Something of a wild shot in the dark here but now that we know all about the Sword, it seems to me Be'lal might deserve a little more credit. It is possible he knew more about the thing, either that it was without a buffer, could be hijacked, also could do the True Power, or all of the above.

Someone, and presumably a Chosen, had to have tinkered with Callandor or gotten someone to do it or it wouldn't have been able to magnify the true power. It's not just that the Chosen ran everything, it is that - according to Moghedien - only a couple dozen people ever had access to the true power and it seems a safe assumption that constructing a a power amplifier for it would require some knowledge and/or use of it. Now the initial suspect would be Elan Morin but then he wouldn't have been all surprised by that ability there at the end. So it had to be a different TP wielder who either flawed the thing personally or assisted/arranged for it. It doesn't have to be one of the 13 Forsaken, but they make up about half of the possible candidates and there's decent odds a good portion of the others were dead by the time Callandor was made since it was crafted late in the war. One assumes at least a basic knowledge of angreal construction and the TP would be needed to screw up the sword and post-tDR Forsaken don't seem to display the former. This is not to say Be'lal necessarily did, but he can't be ruled out and he also could have known about Callandor's flaws from whoever did it. It is also worth noting that Be'lal went out of his way to have black ajah female channelers in the Stone, he obviously thought they were of some use to him, and I doubt their specific battle prowess wa sof interest to him. It doesn't seem too unreasonable that he wanted to seize control of Callandor the way Rand did from Moridin at Shayol Ghul.

Somebody had to make those flaw in callandor, the list of possibly suspects is pretty slim, and they definitely weren't following Moridin's orders when they did it. It seems plausible that Be'lal either did it, ordered it, or one of the other TP wielders besides the 13 Forsaken did and he found out.