[Mark]

About the nail - the blade likely came loose as the wood of the handle aged and shrank, so someone, very likely not someone Burmese or Thai, drilled through the handle and tang and put in the nail. What you found about the tang length is exactly right for a dha. They are very short, and the blade is just held in with a pressure fit, and in most cases an adhesive such as pitch. Taking out the nail and repairing the holes would restore it to the traditional form.

One other thought about the inscription - it might not be Burmese at all, but rather Mon (people in southern Myamar, whom the Burmese call Talaing). I believe the same basic alphabet is used by both, but there are differences in how they are written because they are very different languages. That might explain the odd letters/diacriticals.