I currently have enough visa exempt days to take me up to the day of my flight back to my home country (about 19 days time). I want to join a friend on his visa run to Cambodia which will result in me getting insufficient days (14 days, right?) when I make the overland trip back. That's kind of ok as I am keen on a quick trip to Laos anyways. However, I won't have any proof of onward travel during the 14 days visa free period I hope to be granted when crossing the border, which I hear is one of the things they may ask for.

Should I linger in Cambodia a few more days, fly back or do u reckon it will just be fine at the border if I smile nicely and show them that I have a ticket back home?

Thanks!

At land borders no proof of onward travel is asked for.

At the end of the 15 days you can visit immigration with your ticket out. Based on you leaving within 7 days they can give you a 7 day extension for 1,900 baht (and a passphoto)

they are more likely to ask to show 20000 baht. just bring along your bank statement slip just incase.