I really am so tired of my tired outdated red brick fireplace. I need to know if what I want to do will work. I want to seal my entire fireplace and after letting it sit for 30-45 minutes, I want to apply mortar to it and place river stones all in the mortar. Does this sound like it would work. Thank you

Only if you use a latex modified thin-set. It may be easier for you to mortar on a 1/4" backer board over the brick being that you are using river rock.