Ceramic tile on fireplace seat -- joints?

I am getting ready to do my first ceramic tile job.
Built a fireplace enclosure and I have a base 12" off the ground that I'm going to put porceline 16" tiles on, as a seat to sit on in front of the warm fire.

I had to widen my wood base, to allow the 16" tiles to sit surrounding the fireplace, but, that resulted in a joint all the way around the fireplace where I added about 10" on the sides and about 6 inches to the front.
I have plywood fastened down which I plan to mortar my tile to.
1. Do I need to use some sort of adhesive tape to cover the joints? If I don't will the cearmaic tiles crack there?

2. Will the plywood be ok to put thinset mortar on top of?

Will the plywood be ok to put thinset mortar on top of?

Shouldn't.