" Do we have to open up the toilet side of the wall to install/find the blocking? "

I would.

" Is there a way to install the shower grab bar without finding/using the blocking? "

Not effectively, you will pull the anchors thru the drywall before too long.] if the grab bar is used as a true grab bar.

These wall anchors on steroids have worked fine for us for the last five years.

Saving

I would absolutely want grab bars attached to the studs.

Also I might even want additional grab bars installed so that they are accessible wherever one is in the shower including accessible when one is entering and exiting to help with balance.

The reason why you want them attached to the studs is that they are there for a worst case scenario when someone has literally lost complete balance and is dead weight on the bar. Not having them in the studs might be find when they are used to assist a bit or to steady someone but there could be complete failure when they are being grabbed - especially if someone is larger/weighs more.

Thank you everyone for the comments. @helen, we do already have them attached to studs in two of the 4 walls including one at the entry. Our issue is just with the horizontal one that’s supposed to be on the right hand wall when looking at the shower head. Those monster anchors are tempting, @rwiegand, but we are leaning towards the studs/blocking approach.

These are pretty good, ADA approved and all that.

Wing-It

Thank you, @mongoct. Unfortunately, the company says they don’t work with our Rohl grab bars.

Screwing into wood is better. We didn't find out the blocking wasn't there until after the walls were tiled. Being a trust but verify guy I gave the installed bar a hard hit with my full body weight (substantial, ubfortunately) and nothig moved.