Suwannabhumi has been a junction for many roads for many thousands years. This makes history very complex and interesting. The lack of written records and other evidence (or constraints) invites 'more theories' (or possibilities). I would love to go around and collect more constraining evidence ;-)

สยาม is a westernized word, they say, possibly from สยม (สะยัม / สะอัม / สิ เอม / เสียม) which seems to link to ซิเอม (in ภาษาใต้) and ชาน or ฉาน (in Myanmar - 'ending m' is pronounced as 'n' by Burmese) or อัสสัม (in India) and many other references to 'เซี้ยม' as an ethic of people (in Asia). This suggests many movements over many thousands years of history of this ethic group among many other ethnic groups. Words from many languages are used by people. It is not easy to establish 'pure' core language for Thai. But we can all do our little bits to help.

I wonder if ขอม  is a Pali word (in Theravadi/Buddhist periods)  for Sanskrit กศะยามะ (--> สยาม in Hindi/Brahman periods)? Maybe อาจารย์ ธ. วัชชัย can help here. (I have been looking for good pictures of stone tablets that have these 2 words on to compare the 'letters' -- in my spare time when I am not doing many other things ;-) So far I have no luck.

[If a linguistic connection can be established, then we would have a strong base for ขอม=เสียม(ไทย).]