It seems like the division of the provinces Phra Nakhon and Thon Buri I suspected earlier are just a misunderstanding. I tried to search a lot in the Royal Gazette to find anything about these provinces, but found only very few announcements with any of these province names in their title. And there are many announcements for the same years which have just the two province names, which already made me doubt the existence of the 5 provinces. But I found one which looked like the creation of Thon Buri Nuea and Tai, but thanks to Khun Wisarut at the 2bangkok.com forum I now know it were just two police departments within the province Thon Buri.
But the searching for this announcement gave me several other interesting announcements which I did not find before - the rename of four districts in Thon Buri in 1916, the creation of 25 districts within Krung Thep in 1915, and finally also the creation of the provinces Phra Nakhon and Thon Buri in 1915, which also includes the reassign of Thanyaburi and Pathum Thani to Monthon Krung Kao (Ayutthaya), and the change from the word Mueang to Changwat for the provinces within the Monthon. However one announcement still escapes me - the rename of the province Nakhon Khuean Khan (นครเขื่อนขันธ์) to Phra Pradeang, which happened in June 1915.