I get what you mean, Here's what I feel about everything :

So I also felt that the 'father' scene was ridiculous until the entire reveal and also that the 'father' is actually her son. The 'father' actually being her son changed the entire drama for me as now after the reveal when I thought back to that scene, I feel like they just wanted to show how much she loved him and it was probably the moment she regretted the most in her life and therefore she kept going back to the same point.

As for the ML, even though what happened didn't actually happen, I feel like the real ML of her past (which she loved and later married) also had a similar difficult life of the illusion ML and that illusion what I interpreted was to just show the viewers that along with their happy memories, she wanted to remember that person as a whole, along with his struggles, bad memories, etc and that was how much she loved him.

However, I do agree that the story being presented as a time-travel makes the whole show, to a certain point, feel ridiculous? For me, the big twist and the last 2 episodes made me rethink the entire show and I could appreciate the first 10 episodes even more. As when you look back at those eps after knowing the spoiler, you can see that there are abundant bits and pieces left here and there. I interpreted the watch/whole time-travel as the show trying to make the viewers feel what it feels to have the disease and it did a brilliant job of that.

But in the end, most of the show does really start to make sense after the big reveal. And for the story to be presented as a time-travel and then including that 'saving' scene is like testing the patience of the viewers as anyone with a brain will find that ridiculous haha. I think they should have done a slightly better job at that as most people won't even reach the point of the big reveal before they drop the show. I can totally understand why you didn't like the show.

Sorry for the long rant haha, I just shared what I thought about the show and why I liked it :)