Ive had two pairs of manitous, not shermans but stance blunt's (170mm travel), both leaked like hell, and my mates 'tou dirt jumping forks also leaked like buggery,.
They dont, anymore, actually. Fox's will outlast any post 08 Zocchi, trufact. but i'd buy neither personally Rockshox seem pretty good at the moment
This is going off reviews, from my personal experience and other forums (Pinkbike, aka idiot central)
First 3 would probably fail, topcaps have been done, seen carbon axle washers made before and i think ive seen carbon v brake lever blades about aswell.
Your probably 11 and came on here trying to tell mike poyzer to change your onza from 20" to 19" wheel so you obviously have no clue. + carbon backings would be worth more than £20...
Id be willing to test any carbon parts & look at some FSA & Bontrager XC stuff, they seem to be doing alot of carbon these days. White/Whyte (spelling?) brothers have done a few carbon forks aswell as pace doing them aswell. probably worth a look