Commercially produced coffins are bought all the time by people who have no intention of using them for their primary purpose. We saw one on the show used as a prop at a fraternity house party at least once. As for the marker, once more, monuments of that sort are sold for things other than head markers on graves. The inscription could have been put on later by Xander. It's not that difficult to do. The letters are cut into the stone with a sand blaster (I have a friend who runs a monument company, so I know how it's done) or Willow and Tara could have inscribed it using magic. They also almost certainly placed some sort of ward around the actual grave site to keep it private.
As for the Buffybot, it was shown in the house several times in the first two episodes, which was as long as it lasted in Season 6. Once when Willow was repairing and reprogramming it after a nightly patrol, once when Dawn slept beside it, and once making a huge pile of sandwiches for Dawn to take on her first day of school.
The Scoobies definitely used the Buffybot, not just to help in the nightly patrols, but also to conceal the fact that Buffy, in the role of Dawn's guardian, not just the Slayer, to keep up the illusion that the real Buffy was still alive and healthy. There were several quite valid possible reasons for doing that, take your pick or contribute your own special theory, but, obviously, if Buffy's death had been made known, there would have been no way that, after 147 days, she suddenly popped up once more, alive and depressed.
As for Buffy's financial state, though it was never shown anywhere, it's fair to say that, at least while he was living here, Giles handled most of the financial problems. Once he left, he probably either simply about it, or purposely neglected to send Buffy money. Remember, he went back to England in order for Buffy to learn how to be self sufficient, which would have certainly included financial stability, if not complete security.
We don't actually know that Willow and Tara never contributed to the household expenses, or even helped clean up around the house. If they didn't, they definitely should have. I could see Willow not even thinking of such things, since I got the impression that, whenever she wasn't studying for classes, she was studying one spell book or another. The mundane things like food, utilities, and housecleaning wouldn't even have registered on her radar.
It's ironic, I think, that Giles left Buffy high and dry at the worst stage of her Slayer career in Season 6 in order to teach her how to be independent and self sufficient, when what he really should have taught her was that it's perfectly alright to admit that she really isn't all that self sufficient. It's permissible to ask for help when you really need it, and helping isn't the same thing as doing it for her. That's a lesson they both should have learned, and which the goings on in Season 7 clearly showed that neither one had learned.
