Crazy thought that I haven't seen mentioned, Brandons answer makes me think that instead of switching blades maybe he switched the madman himself. Granted I know very little about the heralds or taln in particular, but his repetitive loop of info and willingness to be kept in captivity by naive people who don't grasp the implications of his return, as opposed to showing a little surgebinding to get everyone's attention make me suspicious of misdirection. But this may be better than hiding his return bc it may allow some chance of getting people to take the desolation seriously.
I'll admit that the end when he catches the dart it made me second guess my theory. Then I thought if it was an act then that was how the impostor became so hyperalert when we know hr had been in a stupor since returning to roshar.
We know worldhopping and various cosmere magic systems r at hoods disposal. Hr shows his mastery of illusion when he tells kaladin the story. He effectively makes him see as he wishes. This could b similar just on a grander scale. Maybe this faux-Taln is roshar's version of a kandra.
I know its crazy, but this mystery is more than a simple switching of swords I'm sure of it.