OK, so it definitely sounds like my corners are pitched incorrectly, which I had wondered about. I'd like to deal with this first. Oh, and I definitely did watch the pitching video on the website, though I clearly missed something.
(not sure how much of this is affected by vintage - mine is a 2007)
1) The corners of the bathtub floor have a loop of elastic cord on them (outside, not the inside floor height adjustment)
2) The tarp has the guyout on it of course.
What I did (as can be seen in fly_stake.jpg - top row 4th over) is stake through the elastic loop on either side of the ridgepole so that the elastic was tensioned. I then did this on the opposite side. At that point I attached the tarp corner guyout lines to the stakes and tensioned.
It sounds like I should have staked the tarp corner guyouts first and then, did something with the floor elastic loops?