Dodi unfortunately was the master mind of all the plans that were made that day. I still don't understand up to this day why they didn't stay in the hotel and wait to go to his apartment in the morning.
I believe that this strange ring was the reason. From the report of Operation Paget I got the feeling that Mohammed wanted Dodi to force Diana's hand. IMHO Mohammed decided that Dodi needed something to present to the public (whoever that may have been for Mohammed, maybe just the family or business acquaintances) after his holiday with Diana. We "know" from Burrell that Diana did not want to get a ring from Dodi, she did not want to take anything that could be presented in the public as commitment to Dodi. Probably she wanted to annoy the RF and Hasnat Khan but she didn't want to be presented as the future Mrs. Al-Fayed either.
So just recall their situation that night. They wanted to spent the evening having dinner at a posh restaurant in the Bastille area of Paris and then back to Dodi's place where he kept the ring. But they couldn't get there due to the paparazzi, so they took refuge at the Ritz, which is half way between Arc de Triomphe and Bastille (IMHO they should have taken the metro, but then....
). Mohammed claimed to have told Dodi to stay there for the night but who knows if this is true? He could have OTOH inisted they go back to Dodi's apartment because of the ring. Both Mohammed and Dodi must by now have realised that Diana was somehow reluctant to accept a ring but dined and wined enough and looking forward to returning to London, she might have taken it none-the-less. So Dodi only had to make sure she didn't give it back the next morning et voilà. I bet the media would have waited in London knowing about the ring already...
But this couldn't happen in the Ritz, the privacy of Dodi's apartment was needed. Hence the strange occurence of the suite at the Ritz: in the morning, when Dodi and Diana first came to the Ritz, they were told that they had no suite available, so they had to go to his apartment, but in the evening a suite was free to enable them to have a private dinner. I guess Diana did notice that, too! Maybe that's why she was upset: she didn't want to go back to the apartment but Dodi (and probably his father) insisted. She probably realised something about a ring would be coming and didn't want it. But when Dodi insisted enough, she said okay.
Just my opinion, but somehow that makes more sense to me than other explanations.
And now Mohammed does not want to tell, that he was the one to insisting on the nightly trip through Paris.
