im sure he would've just snuck around while she was asleep. If he got caught say he was getting water or doesn't sleep much or what have you. Im sure he would have no trouble playing it off.
and yeah, the slide rule had the company of manufacture at the bottom and it's a one of a kind thing so it won't be hard for him to describe the item and then do some social engineering to get the names of who paid for it (Ziegler's men). Once he's got their names he'll be able to track them down and do some good old fashioned torture.
i think he would've done anything to avoid killing her because there's no way Gus wouldn't hear about it, and then Lalo will have blown his cover that he's in Germany. Right now he's got the element of surprise over Gus which he needs to keep in his corner or he's screwed, since he's working completely alone.