As for the post-fire buttplug, it only happens on those routes where Blake regularly accepts anal penetration, so it's not an outrageous thing for him. It does not happen on, say, Vanessa's route.
...
Yes, as of now there's only two ways to escape any of the main four routes: either go on a date with Vanessa, but don't continue your relationship after its failure, or date Liam, then dump him (I guess, you can also add the «date Vanessa, fall asleep at her flat, and never return to her in this list, but that won't let you get on Iris's route).