Plumbing Questions? Ask a Plumber and Get Answers ASAP.
Welcome to Just Answer, my name is ***** ***** I will do my best to help you with your issue. If my initial response doesn’t answer your question then let me know and we can continue our conversation.
While I agree with your plumber's assessment of the condition of the existing shower valve (if it was installed incorrectly it wouldn't have worked OK for 2 weeks), I don't agree with his approach to replace it. We routinely replace shower valve from the shower side using a cover plate (see attached picture). This plate allows for a hole big enough to work through to be cut in the shower wall then the plate covers the hole. The new valve trim covers the center hole. If your guy won't use this approach then I'd suggest you look for another plumber who will.
I have been called away. If you have more questions I'll respond when I return (~1 hour). If you don't have any more questions I'd appreciate a favorable rating.