Fearn - I doubt the tip of the second one has been reground. It is a common shape.
Alaung Hpaya - I'm not really sure about a time period for shagree usage, actually. The ones I can think of off the top of my head do seem more recent, probably 20th C. That bears looking into. IBased on the refinement of the style and quality of the craftsmanship, would say that the second one is early 19th or late 18th century (i.e., before the British ban on weapons in 1868). Not later than around 1868, in any event.
I have the same "feel" you mention for Bama vs Shan and Kachin, and the same lack of a clue for Karen. Kachin dha do tend to be straighter, or completely straight, with that concave tip. I have also noticed that they rarely, if ever, have a fuller as these two do. What little I have read about the Karen, and admittedly I have hardly studied their culture and history, they are not sword makers or blacksmiths, but get their dha and other tools from the Bama or Shan. Among other sources, Ferrars & Ferrars say this, though I find their description of the Karen and their culture to be quite offensive, and rather at odds with ones I have read elsewhere. I would have to dig though my notes to see which other writers mention the Karen's lack of a bladesmithing tradition. Maybe it was Symes ...
I think for most purposes, the Mon and Bama material culture is indistinguishable at this point, and has been for some time. One question I keep trying to answer is, "What kind of swords did the Mon use before they were conquered by the Bama?" After Anawratha conquered Thaton in the 11th C, the Bama almost entirely adopted Mon "civilized" culture, such as art, architecture, and writing, as well as Theravada Buddhism. Mon artisans, craftsmen and scholars were deported whole-sale to Pagan (I found this link to a quicky history of Pagan:
http://www.ancientbagan.com/bagan-history.htm, but Phayre, Hall, Cambridge, etc., are better sources from the academic point of view). I find no reference to an adoption of weapons technology, though, so I am left wondering if the present "Bama" dha in its various forms is an adaptation of a Mon style, or if it is the survival of the Bama style, the Mon weapon styles now being lost. On the one hand you would expect that a conquering people would keep the weapons with which they are familiar, and that brought them to power, but on the other hand if Mon bladesmiths were assimulated into the Bama culture it might be that their preferred style become the common one. I am hoping that the answer lies in the archeological literature & museum collections, sources which I haven't been able to effectively access yet.