I think they could still go for the "Finn was really Slip" idea, but they didn't go enough over Finn bullshitting about himself in TFA. I didn't read the novel, but I wonder if it could still work. It'd be very FFVII-ish, but imagine if FN-2187 (the #1 Storm Trooper) was only awesome in the simulations, and when it came to the real battle he died as soon as he stepped out. Slip realizes FN-2187 can't save him like he always did in the simulations now that he's dead, so he freaks out.
For some reason they mistook Slip as FN-2187 (could have been FN-2187 who did something on purpose, didn't want to go out there and kill people so he faked Slip's death by swapping IDs or whatever, I don't know).
Slip assumes FN-2187's identity, runs away, but is surprisingly really good in this real-world context and becomes the Finn we knew in TFA as a result of having people who believe in him, the usual.
We could find out in VIII that he was the worst Storm Trooper, maybe him boosting the confidence of someone else secretly in a conversation.