I am remodeling my bathroom and plan on tiling my shower and adding a seat in the corner. I am trying to determine the best time to frame in the seat. It seems I should put in the sloped mortar bed, then the liner. Then hang the cement board and put down the second mortar bed. At this point I would be ready for tile. Would this be the correct time to frame in the seat, securing to the studs behind the cement board? Then I would just cover the seat with cement board and tile? Or should I also do something additional with the seat?

Any comment and suggestions anyone had would be great. Thanks

"It seems I should put in the sloped mortar bed, then the liner" so we can assume you are building the shower pan from scratch and this is your very first attempt at a pan, right? I will answer your actually question bit want to come back to the DIY pan idea. The seat is framed before the backer board goes in. I usually do a seat that goes all the way across the back wall of the shower which reaches down to the back lip of the shower pan. The actual seating area is behind or outside of the shower enclosure. Looks like its buried into the back wall. Anyway, after framing them install backer board with a 1/4" p/f slope for drainage. Tape with fiberglass tape and modified thinset and then set your tile. I use pre-mixed stain proof grout but would wait at least 48 hours for the tile thinset to become firm. Here is my advice for your pan, buy a pre made floor, I have seen a very high rate of failed scratch built shower pans built by DIYers. It is not and easy job but this may help How to Build a Shower Pan

I don't actually have enough room for the seat to extend the entire length of the wall. Based on my layout the best I can do is put a seat in the corner. I have seen a couple examples of people adding the seat after the backerboard in up, just wondering if anyone has run into this before.

I would not add a corner seat after the backer was up. It should be attached directly to the framing and not have a backer in between like a spacer.

You might get away with building the seat after backer board and certainly must be before mudding pan. I think you can get away with using kerdi membrane ( they even recommend using greenboard , but I think I'd use hardibacker as the thinset will certainly perform well with the membrane on hardibacker. Than you can( after your seat corner membraned and sealed mud the floor pan accordly. Be sure you do the whole shower tub to the ceiling and understand this kerdi membrane product.

Are you thinking of something like this? Better-Bench by Innovis Corporation

I did a few shower tubs with seats and the pans too with this stuff, here is their link Schluter Systems - Homepage - Schluter-Systems

Ditra is kind of funky but it is great for laying ceramic on a concrete floor. Concrete floors move a tiny bit and ceramic doesn't like movement so Ditra allows a little wiggle room. I would not use it unless setting on concrete because other less expensive products work very well on sub floors.

ballengerb1, as a response to your question with the image... yes, but I was planning on having the seat go to the floor.

My initial gut feeling was to build the seat into the frame before any mortar or liner work starts. So after additional research, comments and thinking I plan on building the seat as part of the framing. So now my question is around the liner I plan on using. I was looking at using the Chloraloy CPE liner. So (1) any comments on this product and (2) do I run the liner all the way up, over and partially up the wall above the seat? And then just build the seat as I would the shower floor, no backerboard?

I would not do it as you describe but then again I'm the guy who said to buy a pre-made shower pan. The seat gets treated like a wall not the floor. It needs backer board that is taped and then tiled and grouted. I have not used a liner in years since I try to go with Swanstone pans so I can't help with the CPE liner question. Swanstone Shower Walls & Floors

I plan on going with tile on the floor, the Swanstone products appear to be a solid surface pan, doesn't look like you would add tile.

As for the seat, after looking at the link (Better-Bench by Innovis Corporation) from ballengerb1 it appears it would be pretty easy to build something like this into the frame. Then I would do as you suggested, add the backer, tape, tile and grout it.