I can only answer you with the facts that have been presented and my opinion.
Diana's decision to go back to Paris was not planned from what we have heard. The engagement seems to have been a figment of Fayed's imagination, his greatest hope, a chance to get back at the Royal Family that he hates but wants desperately to be a part of. Would anyone have taken an engagement to Dodi seriously, after all they had been seeing each other for such a short time, most people would I think, have sat back and waited for this relationship to fizzle out.
What difference would it have made to TRF or government, IF Diana had married Dodi? None.
Would it have mattered IF she married a muslim, NO. The only 'damage', if any would have been to Diana's image.
Would it have mattered IF after such a short 'relationship', Diana had been pregnant, again only to Diana's image.
These are the reasons that Fayed states why, in his opinion, they were 'murdered'.
The security services couldn't have known that they would leave the Ritz at a given time, because it hadn't been decided. They couldn't have known which door Dodi would use, they couldn't know that the paparazzi would not be right there, behind the car, they couldn't know that there wouldn't be a huge stream of traffic going through the tunnel.
The main point though is that it wouldn't have mattered to anyone who Diana decided to have a relationship with.