To my eye the Quilons look crushed and unable to trap an opponents blade. They do not look substantial enough and in addition the blades look too flimsy to even need Quilons. Surely this cannot be a fighting weapon.
It is in fact a court sword. In its secondary role a Badge of Office for the secretariat Officers of Mudalier rank in the civil service. In this regard there appear to be two swords... The earlier Kastane may well have been the Golock bladed similar to Storta weapon seen in the famous stone carving but soon after another Kastane appeared ...The Badge of Office almost Bling format court sword.
The Kastane in its Court Sword garb... was never a battle Sword but a badge of Rank/Office only.
It's tough to reason with circular logic: If you define kastane as a court sword only, it is not surprising if all kastane sensu Ibrahiim were court swords...
Nobody is arguing that the late court kastane was not meant for fighting. This doesn't imply that earlier members of the same sword lineage can't ever have been fighting swords though.
It took its entire hilt form from the Buddhist traditions including the zoomorphic hilt with Buddhist Deities, Peacock tail and Vajra Quilons;..none of which were intended for a Battle Sword.
The Tibetan sword you show clearly has the vajra included into the hilt design. However, I don't think a compelling case can be made that it's the same for any kastane - it's certainly an interesting thought but doesn't fly without a lot more supporting evidence!
A vajra is a 3-dimensional object symbolizing a four-sided diamond; those 4 "limbs" join into a solid tip - sort of like a war hammer. A kastane only shows 2 planes and the 2 mythical creatures don't really connect to the blade.
Moreover, these creatures are obviously taken from Hindu iconography and already have been utilized as cross guards in Hindu weapons. Much of the early Buddhist iconography is based on Hindu roots. Sri Lankan weapons were obviously based on Hindu weapons (certainly allowing for some additional cross-cultural influences, too).
Last not least, the vajra is the icon of the Vajrayana lineage of Buddhism which never had much of any presence on Sri Lanka.
The link to European weapons in this regard is questionable and cannot be attached to the Quilons because by definition the Kastane doesn't have any since it is not a fighting weapon; so why should it?
Let's not throw out the baby with the bathwater unless we have a really compelling reason to do so, shall we? Convincing the mother first, wouldn't hurt either...