When my dad got old and frail, I added that stick on sandpaper to the floor and added a shower chair. He said the vertical grab bar outside was his saving grace. Obviously you want it for comfort for your mom, so do what she wants, but just be aware that the inside horizontal grab bar is set in a way where she doesn't hurt herself.
I'm glad you brought that up because I didn't even think about doing a diagonal bar, but I will have to consider it when planning it out. Right now she thinks she wants a shower seat with a back to offer support so she won't fall backward, but my concern is that it won't be deep enough front to back for her and she won't be able to wash her back.
The shower we got has a textured floor to be nonslip, but I found some mats at walmart that are nonslip and comfortable. I got one for my friend's house and I didn't even need the grab bar because I was no longer slipping every 2 seconds. Before either the mat or the grab bar (suction cup type) were added in there, my brother fell and hurt his shoulder.
I need to make sure to save enough of the cardboard boxes that I can tape them together to make fake walls and set them up so Mom can see how it feels in the space. I might do sort of a dry fit of the shower and add blocking at pretty much ever spot the grab bar might attach just to be on the safe side.
I got a couple more screenshots of my sketchup with some stuff added. The lamp we have there is probably a little smaller and not the same shape, but this was the best I could find. Only working light in the bathroom.
I added the 19" slidebar (but not perfectly) to get an idea of where the handheld would sit). I realized the showerhead has to come down for her to be able to reach it.
I really want her to get a different shower seat than she has bc hers is not adjustable height, is too low, and does not have suction cup feet. So I'm worried it can tip. I want her to get something taller and easier to stand up from since her knees give her trouble.
I still need to figure out how to use fredo's size plugin to make the vertical bar longer as the one we have is 4'.
The shelf grab bar might have to be a little lower so she can reach stuff on it and grab it when standing up, and the side one might be a little higher for her to grab when she's getting in. Her main issue is her balance bc of inner ear damage so it's not like she needs to put full weight on anything or is too weak to get into a sitting position. So long as she has something to grab to steady herself when she wants to stand up, it should be ok. She does better with taller seats. She has much less trouble getting up from the ADA height toilet we put in.