I'm a pretty strict adherent to RAW, and I hate homebrew, but I just can't get on board with this one.
I just don't think the Slow property should apply to using the crossbow if it's "locked and loaded" already. I can see it applying after the first shot, because theoretically not only does it take two rounds to reload and shoot it, but it's slow to reload. But this isn't some sort of 240lb crossbow that requires an Olympic Clean and Jerk maneuver to lift, point, and squeeze.
I definitely don't do much to give my players any slack. I don't "cut breaks". But I genuinely want to see your PCs succeed against long odds. It makes a great story.
But as a DM, no matter what RAW says, I couldn't justify applying the Slow property to a PC with a readied crossbow until after they fired it. So I can't for the NPC either.
But... @dominion451 did pull a good lawyer move here, and I'm willing to concede it for this round since we operate on the principal of RAW, and because I don't want a PC to die in one whack (especially a halfling).
So unless there's any arguements, Slow property applies to all crossbows, but ONLY after the first shot is fired. But for this round Angel and Gilly get a chance to retcon Gilly's injury, because of some great attorney-ship on @dominion451's part.
EDIT: I chose B/X over BECMI because I use the B/X Essentials (Now Old Schol Essentials) books, and because BECMI sort of hoses the Thief Class in terms of level progression (though B/X hoses Clerics a bit, tbh).
But I learned D&D through B/X Essentials. The rulebooks are so well done and played out so well it was easy. So that's why I prefer B/C over BECMI ultimately.