Everything still falls down to being scam. From your explanation you only understood one part of what I said earlier on. For sure, asking for payment in advance may seem to be a scam from the service renders but subsequently the client may fail to send the payment after they have recovered his/her wallet. In as much as we are focusing on the security of the client let us also look at the safety of the people who give their time to recover the wallets. It's not an easy job and it can be very stressful especially when they are giving less details to work with.
You said something that got my attention which is in as much we are considering the potential clients in terms of safety, we should also consider the tech firm that did the recovery, I think you are, some client can be funny they may decide to act funny when they know that the service they needed has been rendered although I believe there should be an agreement binding this and again I think once this service has been rendered there should be a terms that has to compel the client involved to reciprocate the same as the service porvider by giving them the agreed percentage, this is why supported the idea that legal backing will do both the service providers and the clients more good altleast reasons for such idea are coming up.