You may be right, I am only supposing that ticket sales at Althorp increased following Diana's burial there. I do not know for sure.
Since the family chose a private burial at the family home, there may not be any photos or film of the event. If there are, I sincerely hope they are locked away in a vault somewhere beneath Althrop House, not to be seen for decades if ever.
As to whether or not Diana is buried on the island or not, I suppose only the government, the immediate family, and any clergy and staff present are the only ones who know for sure. Frankly, it's none of our business.
We saw one filmmaker's interpretation of what happened that week in the movie The Queen, but even if it was 100% accurate, it only focused on the adult royal family and Blair, not on the thoughts and wishes of the Spencers or the two young princes. And frankly, those were the only people who's opinions mattered. I can't imagine all of the dynamics that were going on that week when funeral decisions were made. She's the mother of the future King. She's not a HRH. She's a public figure. She's a private person. The public wants a huge spectacle. The Spencers want a private affair, and so does the Queen. The media killed her, we need to protect her. No, we need to have a state funeral and the works. No, give her the peace she never got in life. Depending on who you asked, you got a totally different answer.
As distasteful as the topic is to write about, if I had been a member of the family responsible for making funeral decisions, I would have been very concerned with the short-term and long-term security of her remains. We all saw how irrational and emotional people were acting that week - I would fear that some crazy person would break into Althorp's grounds and try to steal the remains - for profit, for ransom, for fame, or whatever.
So I'd be tempted to recommend that the family place her actual remains elsewhere in an unmarked grave known only to those few who needed to know - and perhaps place a marker on the island as a decoy. Then someday in the future when the emotions had calmed, we could always either quietly move her, or even come clean and hold a formal public reburial somewhere else.
I suppose we'll just have to wait and see - eventually these sorts of things become known when the time is right.