Fernando, The figures you note on the Casket are really unusual in that the upper part of the dress is quite Portuguese while the lower part seems Sinhala. May be it is part of a jestering actors similar to the "Kolam" extant dance traditions which still contain elements that make fun of the British raj.
The King in the image included is King WImaladharmasuriya I, alias Konappu bandara alias Dom Joao of Austria- Son of one of King Rajasinghe I's generals who was salin by the King young Konappu escapes to Colombo and joins Portuguese and became excelled in battle under them becoming a captain in the Portuguese army; serving both in Goa and Ceilao. he then turned sides and coronated himself as King Wimaladharmasuriuya and became a formidable opponent of the Portuguese.
I include an image in which the King is in audience with Dutch Admiral Joris Van Spilbergen (published 1605) which is the original from which other copies seem to have been made. The detail in this seem believable and so should be the sword which is quite European. I am tempted to assume that the King retained a European style sword as he was trained in the Portuguese way of fighting and preferred a similar sword.