Just because you mentioned the VoA, i looked up the Thai Embassy website page for it - and true to form it contradicts itself on the issue of how many days:

Firstly: "...You must have a confirmed return ticket to show that they are flying out of Thailand within 15 days of entry, as appropriate." then just 5 lines later it says "If you do not possess a flight ticket to show you will be exiting Thailand within 30 or 15 days of entry you will be most likely to be refused entry

So it's far from clear it seems.

What I would say is that if you scroll to the bottom, the Author does seem to answer all queries so I guess these 2 issues could be put to him