Tub surround question

I've just finished installing a new Roman tub faucet where I had to remove a section of tile and backer board to gain access to remove the old fixtures and install the new ones. The builder did not provide an access panel. My question is: do I have to go back an re-install backer board or can I use 1/2" birch plywood or even drywall as a backer. I already have both on hand. The area in question is 4 6x6 tiles, the surface is vertical and is not really subject to moisture. I hate to buy a whole piece of CBU for this small area. Also, can I use a pre-mixed thinset or mastic to replace the tiles?
Thanks.

I do recommend treating as a wet area. But one way would be to use the plywood , tape the seams with fibrelgass tape and the mastic, and coat with Redguard. Of course a gallon of Redguard will cost about the same as a sheet of backer board! But is easier to handle, and you might later have other uses for the leftovers.

Premixed thinset is ok. Mastic is fine, but technically in a wet area be sure to get a mastic listed at TYPE 1