Tentacle witch one should be pretty easy, just make a decently strong starting character, pick the ability to spawn as deep into the game as possible, stay a virgin, then encounter the witch. Not sure which option you'll need, so save before hand, then try letting the tentacles take you willingly.
For the camp Dullahan, you can find a new location near the beginning of Demon King area that's a post office. If you've become the Dullahan's GF, you'll be able to receive her head as an item. Then simply get high lust, and you'll have the option to use the Dullahan during masturbation