Plenty strong, tile and don't look back, the tile decking is more cosmetic then functional,

Agree. You don't need to demo. But you can add more fasteners.

If you think it'll gnaw at you, just add more cement board screws every 6"-8" on-center. Don't go so crazy that you micro-fracture the cement board and create new problems. Stand on the deck when you screw it down.

And I do recommend screws over nails since the tub is already installed.

Thanks - I know it's strong, I've stood on it before! Not much room for screws, there are so many nails, but I'll try to put some in (wherever I'm not putting a fixture!).

BTW, mongo, I didn't get any responses of the structural nature when I started an earlier thread on faucet/grab bar placement. Since there is a pocket door to the right, only places I have to put the grab bar (19-20" high finished deck and a 30" inseam!) are on the corner of the linen closet to the right, or on the deck itself. Getting a little tight with faucet having to go on front of deck for access (by code), and I really prefer on the drain end (left). I think the handshower is going to have to go on the acrylic "bumpout" in the middle.

So am I going to have to enter/exit from the right? How to put a grab bar there with the pocket door sliding into that wall?

Even a pocket door will have studs behind that drywall. It has to to be able to screw the drywall to something. What size are those studs? Regular 2"x4" turned sideways? If so, then they are sturdy enough, and you can attach a grab bar to them with no problems. You just have to make sure that the fasteners go into the studs, not the space between them.

Studs aren't 16OC in that wall. Got one near opening, next one is 30" back. In between drywall is attached to the tub deck frame from underneath. Header on top.

Thanks for prompting me to look, hollysprings. I was looking underneath the tub deck, I knew drywall was screwed into something right in front of the tub (9" from door opening), I just checked and it does go up higher than the deck (I assume all the way to header), I can see the "break" in the drywall in glancing light and a nail hammered in about 10" above the deck stops. I will have to pull the door casing and jambs to see how thick it is (want to lower the door to the subfloor anyway, they set them up 1" so there is a huge gap under all the doors).

How long a screw do I have to use for a grab bar, just in case it's only 1 3/4" thick (plus 1/2" drywall)? 2" OK?