Just tell immigration that your applying for a retirement visa . It has worked for me last February and in June my mate came with a single flight and we just told them he was probably going to get an extension of stay and that we were not sure when we were going to travel to Cambodia. We did it as pleasantly as we could and they smiled and stamped the passport
you can just go get the stamp and come back there are places that do the border runs but if you go do a couple of days it's very nice and lovely people.
A few times now I've travelled in with a 30 day entry and a return ticket of 120 days and told them I was geting a retirement visa on my trip. The next entry I had a single and a mate had the 30 day entry they asked him about his return and we said he may extend or even go home early. They let him on the plane and into the country. This was February and June this year. I hope this helps you