There are two
questions that matter here.
1) Is Teresa Nielsen Hayden a source worthy of citing in matters
related to publishing?
2) Does Making Light definitely contain material by her?
The answer to both is unquestionably yes. Here endeth the discussion.
Not really. Blogs have no editorial oversight, and their contents are
ephemeral - that is, they can change without notice, leaving evidence
of that change.
What sort of editorial oversight do you believe is in place for an
interview in a normal publication? My understanding is that when a
newspaper or magazine says you said something, then the fact-checker
verifies that you said it, not that it is true.
As to the latter, there's no technical barrier for web publishers of any
sort, blogs or magazines. The main protection is convention; in both
realms it is customary to note changes on the page, and you risk
ridicule if you don't do that. But the original text is not sacrosanct.
To overcome this would it be sufficient in your eyes to cite from the
Internet Archive or WebCite?