I don't know if this would work, but could you sit on the edge of the
tub, then swing your legs over the edge to get to your shower chair?  I
had to do that for awhile after I had my hamstrings lengthened because I
didn't have the strength or range of motion to lift/bend my leg to get
into the tub.  Or try stepping into the tub backwards.  If you have a
sink or wall or something in front of you, with your back to the shower,
you can lean forward on that and then you don't have to lift your leg so
far and there would be less starin on the back.

Another thought...can't your dr. order a home health evaluation (or
whatever you call it) and get a PT or OT to come to your house to help
you figure all this out?  When I had my surgery, I had an OT and PT in
two-three times each just to show me how to get in and out of my house,
the tub, the bed, get dressed etc...Seems like with some professional
help there should be a way for you to stay safe and healthy.

Linda