I did not have proof of onward travel. We did get ask a few questions about our plans. After explaining our plans and that we would be crossing from Thailand into Laos and which border crossing we were planning to use, they issued the visa. We did have to provide proof of funds.
you can't make an appointment you go in the morning, check the opening time. Hand over your paperwork and they tell you when you need to come back. Ours took 3 working days not including the day we dropped it off.
Not sure about a 6 month visa but I was able to apply for the 60 day visa in Cambodia. I have an Australia passport and do not live in Cambodia I was on a tourist visa in Cambodia.