Stephen McIntyre isn't a working climate scientist, but his criticism of Mann's statistical errors (which aren't necessarily relevant to the main arguments for AGW) have been acknowledged as essentially correct. I also took a reasonably detailed look at the specifics of the argument

Did you have a look at these responses? Or at Mann's book on the subject?

There are a number of points here, but the most compelling is that the statistical criticisms were simply irrelevant. Contrary to McIntyre and McKitrick's claims, the differences in principal component methodology make no difference to the proxy reconstructions. And the hockey stick graph has since been replicated dozens of times using multiple different proxies and methods, by multiple authors.