How to deal with errors in well established papers
The most important thing, I think, is that the core results stand.  This means that correcting the errors is not about substance (which might require a new corrective paper or a retraction, depending on the nature of the errors), but is instead about making the road to understanding easier for others who also read the paper.

Since you say the paper has been revised multiple times on arXiv, it seems there is a decent chance that the author may still be tracking on it and be willing to update and correct it.  I would thus start by writing to the author---it's entirely possible you're the first to notice the mistakes, simply because everybody else thought they saw the things that were supposed to be there (copyediting equations is hard).  If that doesn't produce a result within a few months, write to the journal and see if you can get the errata corrected in their version of the paper.

Beyond that, I think there is not currently a high incentive to put in further effort or to put material up on a third-party website, simply because I suspect that most people who look for the paper are unlikely to readily encounter such annotations.  Search engines are pretty useful, though (as this site readily attests), so there's no reason not to put the corrections online somewhere as an FYI to help others who might happen to search for "errors in Paper X."