You’re asking the question from a halachic legal perspective not a non-Jewish law perspective.
What is the common understanding in business when a consumer brings a computer in for an estimate? My understanding is that he’s asking for a cost estimate before he commits to any service or cost he is to incur. Thus neither the end consumer nor the middleman business obligated themselves to you by simply asking you for an estimate. Unless you told him in advance there is a cost to simply render an estimate even if he chooses not to use your services. Or there is a general common understanding that people are expected to pay for an estimate even if they elect not to have the repair service. I don’t believe the latter is the case.
Thus:
1. Yes.
2. Yes. You are holding the laptop owner’s property. You cannot hold his property against his will.
3. No. Doubly so no because even if the middleman businessman who gave you the laptop did owe you money for your service, you have no relationship with the end-user owner so you cannot hold the third-party’s (i.e. the end-user consumer) property for a debt the middleman business has with you.