Regarding the bible:
God. God. Christ. Forgive me.
And when called "Hard working and devout,"
You, they didn't tell you about… About me? Shit, I mean, I, I was? I am? I am those things. Or was? I don't know anymore. God.
The date the bible is returned is shared by the denial of the butane lighter request. She was going to burn it, at first.
All these are designed to show that she's got some major conflicts with her personal faith, even though she unilaterally hates the church. She was raised her whole life in it, more or less, and it's ingrained in her. She eventually asks for the bible back as a sign of her trying to reconcile herself with her personal faith.
It's also worth noting that the weaponization of the Douay-Rheims Challoner Revision in the journal excerpts further concrete the idea that it's personally significant enough to not abandon. It was what was on hand, yes, but it's also reclaimed.
Regarding the semen:
I made a deal to make him pay. For a price, of course. But I was, pardon my French, a fucking slut back then. Fucking horrible. Christ. Christ. So, so I didn't care. It was a fucking upside to me.
It's a Chekhov's gun alright. The collection of semen as relating to a succubus, and a concrete signifier of the symptom of promiscuity arising from sexual abuse. It sticks with her just like 4166-2.
To be clear: she doesn't need it for sustenance. She needs it or she'll break the pact and likely die.
Regarding the priest:
Deliberately ambiguous as to if/when he died. 4166-1 "didn't even do that right."
Regarding 4166-2:
SCP-4166-2 manifestations are to be terminated upon sight.
The curt and simple delivery of this implies methods of terminating humanoids apply, namely firearms or blunt force. Added a line to say it demanifests instantly.
As for how they react to being attacked:
If SCP-4166-1 retains the memory of containment of SCP-4166-2, SCP-4166-2 proceeds to take action to counteract previous methods of containment, up to and including manifesting with weaponry.
As for "what happens if -1 cannot defend herself from them." there are two answers.
The first is that the foundation couldn't know what happens because they won't let it happen.
The second is one I won't write. You can put the pieces together yourself, but I refuse to write it.