I think we need to have a deeper appreciation of what the "Mahanikai" really means in the context of Thai ecclesiastical laws. It's a melange, and not a monolithic grouping.
It's not a Vinaya innovation, but a product of Thai civil laws. Beginning with King Rama IV, a civil law distinction was drawn between (i) the new lineage that centred around Wat Boworn (specially built by Rama III for where the Prince Mongkut practised) and (ii) the "rest". The "rest" was just classed as "Mahanikai" for convenience. I don't think it would have served any of the "State Buddhism" agenda of the Bangkok royals to identify each Mahanikai sub-type (eg the Laotian lineage, the Lanna lineages, the Lopburi, the Nakhon Sithammarat etc), not especially when traces of the old kingdoms would have been connected to these lineages which had some "national" affiliation.
But, perhaps after a century of enforcing this artificial distinction, the differences between the various "Mahanikai" sub-types would have begun to look less pronounced. Yet, I still see some persistence, particularly in the regional iconographies, where "preference" for certain iconographical features of the palladia of the old kingdoms show up in how the Buddha images are cast.
In any event, I even suspect that much of Thai Buddhist lore cannot be ultimately accounted for by a pure Theravada provenance. Think of one of the 3 variants of the "Monday Buddha", and you'll have to dip into the Sarvastivadin Agamas for that legend. Likewise for the Phra Sangkachai myth.