Was described as a "senior administration official." Which could be anywhere from a department head in the White House up to the VP, including on the way, more senior WH staffers and cabinet secretaries. I agree it wasn't likely Pence was someone lower on the food chain, but its' also someone whose name we've at least heard before. I also think this person is very likely a source for the Woodward book, and possibly also the Wolff book.
This editorial is arguing that NYT would never have published the letter had it not come from a major official.
https://www.cnn.com/2018/09/06/politics/new-york-times-op-ed-donald-trump/index.html
I don't know if he's right, but his argument sounds plausible.
No, it's the op-ed page, so I think that's wrong. The standard is more lax than the newsroom.