Thai script and its past

According to the official version of Thai history, the Thai script was developed by King Ramkamhaeng (reigned 1279-1298) of the Sukhothai kingdom. The earliest inscription in Thai script by King Ramkamhaeng was dated to 1283. There are several versions about what could have influenced or inspired this script. A popular version says that it was influenced by Kmer script, a linguistic version says it was developed from ancient Brahmi script of South India, other historical accounts hint that the Mon culture and probably language had a strong influence, and finally there is the often overlooked Lanna script. In preparing my paper, trying to uncover the influences in the development of the Thai script, I took a closer look at the history of the Lanna Kingdom, which during the reign of King Mengrai (1259-1317), founded its capital in Chiang Mai, in the northern region of Thailand. Apart from the well-known story of the famous alliance between King Ramkamhaeng, King Mengrai, and Ngam Muang ruler of Phayao, little credit is given to Chiang Mai-Lanna's influence (if not) origin of Thai culturre. Most history books seem to gloss over the fact that it was this alliance with the main military defence provided by King Mengrai that stopped the Mongol invasion into the region protecting the southern kingdoms. Despite all the confusing historical accounts, the events of this 13th century was an obvious turning point in Thai history. Thai kingdoms and city-states in the geographical location of present day Thailand were melded together whether by force or willingly, giving birth of the Thai "nation".

My research gave me the following information about the scripts that had probable influence in Thai script:

The modern Thai script
Brahmi script, 5th century BCE
Kmer script
Modern Tai Lue script, which gives an idea of what Lanna script looks like, Tai Lue script has been used since 1200 AD in Southern China
Mon script
I also found this interesting outline of the Mon script family tree, which despite its linearity gives a good picture of the development of scripts in SE Asia.