Shower – What backer board for Kohler Choreograph in the shower – osb or greenboard

Greenboard is water resistant, but should not be used in areas that are going to stay damp, such as tub surrounds.

Once water gets into it (especially if it's not sealed correctly, and there's not the requisite gap between the tub and the wallboard), it will slowly deteriorate and/or mold -- it's water resistant, not water proof.

Yes, cement board is a pain to work with, it's better than having to tear out the whole wall because something was off in the installation of the greenboard.

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All that being said, I believe I've seen some home improvement shows that used greenboard and then waterproof membrane over everything to add an extra layer of water protection when they were doing a shower install (tiling the walls and floor; and there was no tub they were joining up to).

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You might also want to check with your local government -- it's possible that there might be codes that dictate what you're allowed (or required) to use in shower/tub areas.

The best thing you could do is to put ceramic tiles but since that's not what you want let’s try to think of something else. Firstly you must replace damaged drywall. It is extra sensitive when it comes to moisture or water. You can, and should replace it with moisture resistant drywall which is normally used in bathrooms. Then you should apply some waterproofing layer; something cement based (something like Sikalastic-200 W).But thing that is worrying me is do you have horizontal waterproofing layer (waterproofing on your floor? I'm asking this because horizontal and vertical waterproofing layer should be continuous. Than you can pain the bathroom with some exterior (façade) material which is impermeable to water or you can use something like travertine technique which is more expensive but looks really nice and it’s waterproof.