I think it would forcibly have occured to Aginor to test all the Shadospawn they made for the ability to channel...

If the Fades had the latent ability, Aginor would have found out, especially about the Myrddraal as they were a mystery - something unforeseen, he's put efforts into solving.

That's not to say the Myrrdraal may not have a kind of latent ability, but I very much doubt they're human enough genetically for the ability to be sensed by humans, and to work.

It's logical. The ability isn't random. For it to show up it would have to have been originally intentional, starting with Aginor trying to make the Trollocs a channelling species originally and therefore using the right human genes for the ability to manifest (thus all Trollocs would theorically have the genes). And forcibly he would have tested all his creations. Clearly if he tried that path, it never worked, the genes are there, but the ability is stunted. He says so himself in LOC, that Myrddraal have strange talents, but not that one - he's completely mystified SH can channel, and this strongly suggest he's tested the Myrrdraal for such an ability.

And it still doesn't explain how Myrrddraal travel through shadows like the Eelfinn in TOM do, or that they move like the Aelfinn do (they also sound like them...), or why they don't seem to obey the same physical laws as everything else purely from the human world.

By the way, it's too bad we didn't have a later POV with Aginor's further musings, but the Chosen seem to believe Shaidar Haran isn't truly a Myrrdraal. IMO, Shai'tan transmigrated the soul of a human channeler into a Myrddraal to give it the ability to use the TP and make it a physical vessel to use.