There are several options in the dialogue to rescue Lae'zel, I can't recall have a problem but I've only done a handful of times.
I thought you were referring to Lae'zel's character as a monster rather than monster as a technical term. I've never had Lae'zel leave me in the intro cutscene, in fact I don't even know what cutscene you are referring to. Shadowheart assumes that Lae'zel just walked off. Shadowheart is unconscious so how would she have known?
I can't remember where or when it takes place but you have a conversation with Shadowheart about the artefact now being in your backpack. You offer to return it to Shadowheart but she says the artefact seems to have made its choice.
There is an area transition to the Mountain Pass which gives you a warning about tying up loose ends etc. The gith patrol is still Act 1.
