@Baksz we know from the early chapter when lacile and leana left, pipi and meme(lol) is also there. so it is possible for 2 of the same person to exist in the same timeline.
by that logic, the current leana and laccile duo can wait until 10 years in the future, go up to ariadne(stay in the city when the landmass go YEET), and exist at the same time as the current leana.
yes the reason she went back is to fuck up her father. but there's no guarantee it will succeed. maybe they fail and now they want to help the current lacille and co.
and yes, by that logic, they can make the lost 11 army.
to clarify, according to my theory, there's 3 set timeline.
1. laccile, pipi, and meme, etc2 timeline. the original
2. leana and ariadne knight's(10 year into the future who hopped down).
currently the party is a mix of these 2. and the intruder in this chapter, in my opinion is:
3. future lacille, leana, and lost 11 (the current party who failed went up to ariadne)