Hi Bayla, Welcome
Usually, it's better not to mix the sealers. All you need to do is pour some water on the sealed grout and see how long it takes to start to darken. If it just beads up, and doesn't darken, you'll be ok. If it darkens quickly, then you might want to consider using a sealer removal product, then seal with TYW Stone Armor.
Seal the shower with what same product?

Probably yes.
Tile Lab will work, but will more than likely need several applications to get the same results. The product is made with less solids (therefore needing more product applications to achieve similiar results).
I highly recommend caulking with silicone caulk or siliconized acrylic caulk along the tub.