Stevia is fine.
A little unsweetened cocoa is probably okay, but that can add up over a week’s time, so limit it.
Same with the broth: if it makes the difference between sticking to the diet and not sticking to the diet for 28 days, then those things are okay. But ideally, you’d avoid all calorie-containing additions, even if they aren’t “bad” for you.
The V-Diet is not just about fat loss, but taste/craving changes and bad dietary habit removal. Part of that comes from its strictness. Make too many little tweaks and even if the body fat comes off you may not get the behavior and taste changes that make the diet so effective in the long term.
So, tread carefully here.