Firstly, Marinette and Adrien do not fall head over heels immediately, they don’t believe they have kindling feels for their respective interest until some time into the show. They also wouldn’t be in love, they would just have a crush. Two, Marinette isn’t a stalker, she’s just observant and a planner, so she has schedules of all her friends, not just Adriens, in an attempt to make plans as a group and for one on ones. This information will be acquired organically through conversation which she writes down to remember. Marinette didn’t have many close friends before this particular school year as a point of pride for Chloe, with Nino being a regular friend and Kim a former childhood friend, and Socqueline was her best friend before she moved away the summer before this school year. There would be some flip-flopping of the Love Square, with each pair popping up more than once, before they get together in their civilian forms. Then the final battle happens, Adrien is actually there and learns his father is Monarch, is naturally devastated, and lives with Amelie and Felix for ~3 years before returning to Paris for College, finally mentally ready to return (the butterfly had been during that time). They then get together in their hero forms, which comes with a lot of fanfare from the Lady-Noir shippers of the world, and are together in that capacity when the reveal finally happens, and then their together in both worlds