Okay: I managed to find a (thankfully very brief and straightforward) walkthrough. After completing the very next step of the quest that way, I managed the rest of it without need for the walkthrough. I'll probably reply to this with a full review later in hopes that such criticism may help future updates.
I found it by looking on the Fandom Wiki: https://ldp.fandom.com/wiki/Lust_Doll_Plus_Wiki
Each quest has a name, just check for the quest you're looking for.
For the specific quest, that's here: https://ldp.fandom.com/wiki/Don%27s_Downfall