Hi Ed
Scented or other wise hampered tissue paper is be avoided. Normal tissue is fine and a lot easier to get a hold of.
As for shirasaya, koshirae can be very old and the insides get a bit dirty and messy from years of oil and and wear. They are very hard to clean (near impossible) so a shirasaya is used to prevent the owner putting his / her sword into something that could damage the polish. Shirasaya can be split to be cleaned and are easier to replace than a full koshirae.
Also a sword in a koshirae can add stress to the koshirae that is possibly quite old and fragile. Imagine the koshirae as a tuxedo and the shirasaya as Jeans and a T shirt (or pyjamas).
I find that swords need very little oil and general mainatance. I live in Tokyo and I think that the climate has something to do with this. I oil my blades about twice a year when the weather / seasons change dramatically. I think that if living by the sea (salt air) you would need more oil and more checking for colouring on the blade. With a sword, only a very thin coat is needed and very little oil should be used. If you find the blade has beads of oil on it after oiling then you have used too much.
I think that a blade should always be kept in a scabbard be it koshirae, shirasaya, a roll of newspaper ... Murphy's law comes into play. Nihonto are very very sharp. Pets, children (two legged pets??? :lol: :lol: :lol: ), a bottle of whisky later, earthquake, walking about in the dark, tripping on the mat etc could lead to the unthinkable. Also swords are strong but a polish is very fragile and the wood of koshirae is said to let the moisture out but not let it in. The scabbard is basically a protector for us and for the sword.
Just my thoughts