I kinda wondering if shadow will return to respawn spot or be at the location I last left it at.

As soon as you mount another horse, your current horse returns to it's original location. So Shadowmere should return to his pond as soon as you used Avrak.

But don't take my word for it, keep a Shadowmere save backed up, cause I did hear about a guy who lose Shadowmere to this expansion as soon as he installed it.