Hi Valjhun,
I do understand if Indian weapons catch you, they are fascinating
.
You can use W40 when cleaning the weapons, but I would not use it permanently, to my opinion you should use sewing machine oil. It is a neutral oil and very good, but for preserving I would use solid car wax, I have used it for many years, and it keeps longer than oil.
There can be several problems trying to preserve koft gari, on is if the rust has gone under the gold/silver inlay, if this is the case I don’t think you can do anything, but maybe someone else, who knows more than I can give a suggestion to what you can do.
I can't help you about laque work - sorry
Best of luck
Jens