just glue it together. the only reason to pin anything in place is if it is ONLY held in place by glue; being a stick tang, the tang itself goes through the guard and so it can't fall off
fibre liners are purely decorative in my opinion. They may have some vibration dampening effect if you insist on hitting your knife, but I'm not sure myself. As for the prevention of these corrosive woods coming into contact, that is true if you don't use any glue at all, but the water will still get in there. Glue on the surfaces of the pieces will create a hermetic seal between the metal and wood
Also, the fibre is very slightly porous though not as much as wood of course.
Thin leather works fine as a shock absorbing gasket, it's been used on things like chisels that are struck for many many years.