One last point to contribute here is that there -are- occasions where writers will use self-insertion characters in order -to- mess with a character's life. Either with the pseudo-romantic idea of becoming this character's healing influence and support, or by virtue of empowerment in that they have a character who -is- capable of so thoroughly derailing this strong central figure. The nonconsentual sex reinforces this idea, with her showing dominance.
Not to imply that this -is- what Devin does with Tarantula necessarily, just that it -is- a possibility and the fact that Tarantula has been a hassle to Dick does not immediately negate the possibility of this being a Mary Sue-as-fulfillment character.