Nicely stated, and I would like to note that my intention was not as much correction as clarification, and understand your notes toward research were more to your placement in coming into the discussion. While you have noted you are relatively new to the research on these weapons, I commend you for your enthusiastic participation and would point out that here we are all students on arms study in varying levels and degrees, but all working toward the same goals, learning together.
You are asking well placed questions, some which have indeed been somewhat travelled in other discussions, and some which have not. Your note on gems is interesting, though I am not certain that this complex aspect of decoration and often talismanic properties would be pertinent to our objectives here on the kasthane.
Good question on the archaeological aspects, and indeed there are numerous studies on Sri Lankan sites as well as the iconographic sources.
Deraniyagala , 1942, p.111) notes, "...the earliest swords appear to have been straight, leaf shaped and double edged, resembling bronze age weapons. Their shape suggests they were mainly offensive. A subsequent development was the wavy blade, and ultimately the curved scimitar type which increased enormously the power of a slashing cut, but sacrificed efficiency in thrusting or parrying (footnote: the scimitar type is probably of Arab origin). "
It is further noted that the early swords depicted on the Tivunka Patimaghara (misnamed Demala Malia Sigiriya) at Polonnaruva are of the leaf pattern and that others from Sigiyira in the Columbo Museum and of the 5th century are single edged and long, straight and double edged. Other excavations of 12th century site at Polonnoruva revealed long straight single edged blades with obliquely truncate point.
It is noted that these are some of the types depicted on frescoes and paintings up to the 18th century "..when the curved, scimitar shape with the lions head comes into fashion".
These interesting entries indicate of course that the early swords of Sri Lanka over a very long time were functional and decoratively austere until relatively recent times. Clearly the 18th century date for the lion head hilts is not quite correct as we are already aware of earlier examples, however I believe the author is referring to the ceremonial sword of rank rather than likely earlier forms bearing some degree of traditional iconography.
Typological chronology is of course an ideal goal, but one which is particularly elusive in the study of most histories of ethnographic forms as reliably provenanced examples are rarely available. Even iconographic sources are sometimes unreliable as artists often tended to be anachronistic in their work as they depicted events of earlier times. As you engage further in this and other studies of these types of weapons it is interesting to see how these kinds of discussions do benefit comprehensive knowledge by compiling data . In these cases, seen throughout the archives here many impressive results have been accomplished.
While you may be a newcomer Napoleon, you ask great questions and follow the course, and its great to have you with us on the adventure!
All the best,