After a careful rewatch, I don't think it's only a bad dream, but a real time-and-space alteration. It has to be cleverly disguised as a nightmare to pass the censorship. My theory below... [spoiler]
To make it easier to understand, let me divide the alternated time character version as Past ML and FL, Present ML and FL, and Future ML and FL.
After Present FL died being stabbed by Li Ning at the end of Ep 15, the mourning Present ML decided to alter the time and space by calling the Future FL. Ni Ning warned that if he does that, the whole memories of him and the Present FL will disappear. Yuan Xia will only be remembered as Wei Xi Qi's former assistant whom he fired.
It's a tough decision, but Present ML called Future FL, who was just fired in 2024. He introduced himself as Brother, he told her what to do. Present ML then collapsed. Future FL did what he asked, and added a jar of candies and an encouragement note next to the phone.
Future FL then flew to Paris, Future ML confronted his uncle. He also said Xi Lin and Xi Kun would be the next turn. He must've known about their involvement through his phone convo with the Present ML who explained everything.
After all issues are resolved and the bad people are arrested, Future ML threw the phone away at the beach. He also said he's the luckiest of them (the 3 versions of MLs) since he and Future FL only shared good memories. No more need for time altering because both ML and FL in any universe are safe. The phone is destroyed so no other greedy people can have it.
As the time-and-space alteration device is destroyed, there won't be any more 3 versions of MLs and FLs. They're all just one ML and one FL. What happened to them?
1 year later, Future FL came back to China. Ni Ning told Future ML where Future FL lives and he goes there with the jar of candies she left in the attic to let her know that he is Brother. He also called her by her nickname only their young version knows: YuanYuan. She then knows that her former boss is Brother.
We see Past ML, who is paranoid and has PTSD experiencing several deaths of Past FL, woke up and is consoled by her. The fact that he sleeps with the marriage certificate under his pillow means he has indeed gone through all the bad stuff, he's been clutching onto the tangible proof that Past FL is indeed alive and they're married.
We don't see about Present ML and FL. But we know after Present ML woke up from fainting he saw Present FL's tomb was gone and so were all the mementos they had. The only thing left is the young FL's school badge. It means all things that happened in their time and space never happened. But since Present FL's tomb is gone it means she's not dead. But where is she? Paris, of course.
The ending scene shows their marriage certificates stating that they were married on 14 July 2025. In Ep 15 the last we saw Present ML and Present FL together they were not married yet since Xi Lin showed up before Present ML and FL's wedding and stabbed Present FL to death. That means that after her tomb disappeared, Present ML finally met Present FL after she was back in China and they got married.
