Tiling For Shower Over Bath Project

[Ben6279]

I'm about to refresh the bathroom and install a shower over the bath. The house is nearly 10 years old with plasterboarded stud walls. Today I removed the extractor fan to give me an idea of the bathroom wall construction as from in the loft I can see it has a lining of OSB in addition to what I would presume moisture resistant PB. Once the fan was removed it seems my face surface is PB (MR?) with two layers of 18mm OSB onto the studs with insulation between.
The bath has only been used for that with no signs of ever having had a shower and tiled 3 rows up at both ends & side.
Despite everything feeling solid & intact I'm saying to myself "it's been up 10 years and would take 6 boards of cement board to replace". Now am I right in presuming that:

• a: this is the correct attitude to approach this with &
• b: would I just be needing to remove the PB back to the 36mm of OSB or be removing the OSB too going back to the studs?

I obviously want some shelving but didn't want shelving protruding into the showering area so I was hoping to cut some shelving niches into the stud wall along the length the bath and opposite end to the shower. I've gathered tanking will be required in these areas but am I right to believe tanking / priming the remainder of the cement board which is intact is not required? For the joints in the cement board where they meet / corners do these need tanking or is jointing tape sufficient?
Tile size I'd expect to be maximum of 500 x 300 if that.

Thanks