Interesting topic. I often wonder if foreigners struggle with the 'written' pronunciation vs 'conversational' pronunciation when trying to learn to speak Khmer.
I'm Khmer. Born and grew up in PP until I was 12 so I can still speak pretty fluently, read pretty well and can write a bit.
I'm not quite sure when and why the substitution of 'h' for 'r' in the 'conversational' pronunciation started. Cambodians (not just people in PP) have always pronounced words very differently when conversing than when reading. I'm sure the fact that it's easier on the tongue thus allowing them to speak faster is a factor. I also think it has to do with being more casual when having a conversation with someone rather than reading out a paper/book in school or office.
I don't consider this to be an accent because it is a way of 'simplifying' the pronunciation for lack of a better word. It's spoken this way everywhere, not just in PP. This is not to say however that some would try and pronounce words the proper way when speaking to foreigners to aid them in understanding. The assumption is if you were to hire a teacher to teach you Khmer language then he/she would teach you the proper pronunciation.
There is an accent between PP and Siem Reap. I personally don't hear a huge difference but probably because I am somewhat of a native speaker.
In terms of overseas Cambodians speaking Khmer, they tend to have an 'accent' if they were born overseas or emigrated when they were really young. So they would be spotted a mile away as a non-local. I'm pretty sure though that they don't even need to speak for the locals to know that they don't live there; just by what they wear and their demeanor. I speak pretty fluently (except for new slangs which I don't understand) without an accent but they can tell that I'm not local.
Hope that helps...or not