Sounds pretty good from that. Tho I question why it seems Sera can dictate what you can do too her. Why wouldnt Mars whos in control take her VV whenever he wants? O_OA lot of stuff has added since your last playthrough, so answering your questions:
Yes, Cecille unleashes her evilness and Erin turns out to be the main victim. She mainly humiliates him and without Mars she turns out to be even meaner. You can even allow her to feminize him but I have a guess that you don't care for this kind of content, so you can refuse if you don't like femboy content.
Currently Mars has already had sex with almost everyone in the party, the exceptions being Sera (you can only do Anal with her, vaginal is only if you choose her latest route ending) and Erin (that you can also fuck if you choose the femdom route)
It is possible to have sex with a LOT of NPCs from the village too so if you like depraved stuff and destroying families by turning wives into sluts and husbands into cuckolds, you'll probably love the NPCs content.
And yes, Mars accepts Hellele help for most of the things, but he's still cautious with her.
As for hellele, yeah I know it was early on. but I got the impression that was the path Mars could become a real demon (he was half right?) and then kill the demon lord and take his wife, That cute demon that kept visiting us. I dont really se any other path how he could defeat the demonlord, other than if he used the his corrupted heros /shrugs. Under the impression he'd turn the girls into his sub-ordinates or harem.
If we are actually turning Erin into a girl, it be quite interesting if he was to keep his hero powers. To be used for the MC that is.
Will try it out sometime.