Also note that mithral counts as silver for purposes of overcoming silver damage reduction, but doesn't indicate that it reduces a weapon's damage. It is a fourth option for overcoming silver DR, though a pricey one. Mithral is a Core Rulebook material, so may be available in some games where supplements aren't allowed.
Silversheen is likely both a special material and a wondrous item because someone didn't check to see if the name was used already. Just consider them two different items that share a name, as that's really the case.
The advantage of the "silversheen paste" (likely a better name for it than sharing the name "silversheen") is that it can be applied to whatever your weapon is, so if you have a +2 keen frost cold iron battleaxe, you can pute the paste on and it acts like a +2 keen frost alchemical silver battleaxe for the duration of the encounter.
The paste allows you to keep your weapon's other properties and only change the effective base material, instead of having to carry a second silver weapon and get it enhanced with a bunch of weapon properties as well.
Actually, I think I like the name "silvercoat paste" better than "silversheen paste," but you can call it whatever you like. The point is they just messed up and named two different but similar things the same name.