If your save slots are causing you trouble, I'd recommend deleting all of them rather than saving over them. Hover over the slot with your cursor and hit the delete key on your keyboard. That's the safest way to avoid possible issues.
The guide is a little bit tricky to avoid spoilers, haha. It says you can 'consider helping the guide'. Not that you 'must help the guide', like it says in Margret's section. So, yes, when it comes to Lu, there's something else that can be done~
If you need more of a hint, let me know! Good luck having everything work out :]