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Yep @skinny's right, the ladder will be rolled up in a cupboard under the sill so we can just chuck it out the window if needed.
Spxtz-
If the box is under the sill and the slings will be running over the frame then the anchors will see very little load. I would suspect that they only get about 50% of the load, so one 80kg person climbing down a ladder will only put about 400N of force on the anchor, shared between the two. Overbuild it for sure, no point skimping on a safety feature, but just to put your mind at ease that whatever you do will be plenty strong enough, you've clearly thought about it. I would possibly suggest that having a simpler, non fixed mechanism, like a brace which pulls on either side of the window attached to the ends of the ladder might be better for something which will hopefully never be used? Something like this, which I'm sure someone must make a specific thing for window ladders:
chiroshi
The Petzl bolts would be bomber, plate fixings in wood should be fine with some proper screws.
I'm not sure if I understand the setup completely, but I think I would be more worried about the slings and or the rope on the ladder deteriorating due to weather and UV