Was there any reason for Nixon to be in the story, though ...?

And wasn't the future Doctor, ahem ... dead by the point the invitations would have to have been sent? Therefore he can't have sent the invitations, unless he was planning to die. But as his younger self wasn't there by that point, how could he have known where and when and by whom that death was going to occur? Unless he told the person who killed him where and when to be, but then how does the spacesuited person travel in time anyway? And why do they appear in the water? And under what circumstance would the Doctor need to have himself killed - and I mean killed outright?

It strikes me as yet another piece of woolly thinking from Steven Moffat, hidden under the disguise of "cleverness".