How often does an O ring on an underwater camera housing need to be replaced if

If you have difficulty sourcing that O-ring, yes you can deviate in material. Changing the material may increase or decrease the performance of the O-ring. Ironically, we don’t know what the original design was actually tested to. I believe the specification stated was 4 atmospheres which is approximately 132 feet however a good design has additional safety buffer. And in reality most sport divers have no reason to venture to that depth and the probability is they have a much more exotic camera if they do. So the conundrum then becomes what do you want to accomplish based on your actual application.

With regard to the lubricant, silicon grease is ubiquitous and for this application there will be no reason to deviate.  

Rick