Regarding the warped valve cover: This is also a problem on 126 V8s and though I have not personally done it, someone else on this forum took his and had it machined. So you may want to take your valve cover to a machine shop and they could just mill off what they need around the bottom to make it true. I'm afraid the sealant would be more of a band-aid fix.
'86 420SE Euro
904 Midnight Blue, Gray Velour
Dad bought it new, now I own it.
"A Mercedes-Benz is like a fine wine, it only gets better with age."