I so wish your car was AMG motored -- if for no other reason than to shut up these naysayers -- but it's not sounding good.
If you can't remove the valve covers or take more detailed pics, perhaps just a video of the car running and revving?
I don't consider myself a "naysayer". Claims are being made that directly affect the suggested value of the vehicle in question, and having looked at several 500SELs of similar vintage in my area recently, I want to know what, in fact, establishes that such claims are valid.
There has been a fair amount of information provided by other members that has been quite helpful in establishing the credibility of such claims, whether they pertain to this car or others. As a result, I now have a much better idea of what to look for to determine if a car of this model and vintage could potentially have a pedigree as this one is claimed to have.
The "naysayer" attitude that you may perceive is, I suspect, more due to the seller's unwillingness (to date) to substantiate the claims, or at least conduct some of the inspection necessary to do so. Some of the things suggested that would put many of the questions to rest are quite simple and require minimal effort. So why not do it?
It falls back to the old line of "the AC just needs a recharge". How difficult could the be to undertake? The unwillingness to exert minimal effort to resolve an issue speaks to credibility.