Seems like something that's completely up to the vixen, really. She's the one having sex, and it's really up to her if you're allowed to be there at all. That being said, I think maximum cuckquean involvement is best, if the vixen allows it. You should be doing everything in your power to make the experience as pleasurable and comfortable for them both as you can. Acting as a pillow or backrest for her, licking her clit while she's getting penetrated, whatever works. That fourth image in OP, that's something that should only happen if the vixen says it's okay. If you've got time to rub your clit, you have time to rub hers, instead. Sort of like "If you've got time to lean, you've got time to clean", but sexier.