So, when I get up to the Fei part I can't decline her offer. I have 100% Bothrioc Oil addiction which is causing me to refuse Charles' fix. Is there another way to lower my addiction so I can complete the quest or am I stuck being an egg slut?
No, there's already a thread about this, they've created a work-around. However, I don't know if it's in the Public Build yet, so you may have to wait.
Well, my first answer would be "Don't become sexually addicted/helplessly subservient to the race you are trying to act as a diplomat for" But that's not really helpful. The lack of ways to become *Un* addicted is a bit of a problem though, yes.
There is a save edit solution I found to avoid the problem. You have to erase the variable meaning you are going to join the harem, after you talked to Fei but before you report to Ara Kei. The game shall then interpret the situation as if you had successfully employed the alternative available at less than 100% addiction (provide Fei some new genitals). Search for the other thread for further details, such as the variable's name.
No, there's already a thread about this, they've created a work-around. However, I don't know if it's in the Public Build yet, so you may have to wait.
To my knowledge from discussion in that thread, the workaround only works for less than 100% addiction. If you have full addiction, you are SOL, and the devs blessed using save edit to escape the trap. If they have changed their stance, and allowed using the alternative with maximum addiction as I suggested, it is a welcome novelty to me.
I feel like if reaching that point dooms you to a bad end and keeps you from interacting meaningfully in other ways with the faction in question/prevents you from completeing sidequests AND disables your ability to reduce the addiction level then it should just give you the bad end as soon as you hit 100%