its not about how many times they need to change the Inlay but Solve the problem instead. This is really horrible! I doubt it's Alluminum because Rolex and other watches used Alluminum inlays for decades and never had this problem. I think it's a different metal used her. Maybe zinc or some cheap crap metal. Aluminum won't be eaten away like in no time.
If it comes to me I would throw the watch in their faces and oblige them to solve the problem. That's really not your problem to worry about this and see how to fix it but theirs. Go back to where you got it from and tell them to do it. It's a diving watch after all. What the hell?!
Laurent