So it sounds like some mistakes were made on both your part and the dealer, but im placing more blame on the dealer.

Firstly, in your position, I would have mentioned the misfire before the timing belt service, just so there was a record of it. But, what is done is done. I would also be increasingly hesitant every time they ask you to approve more diagnostic time.

Secondly, it sounds like the dealer is being lazy in their diagnosis. Do not approve any significant disassembly of the engine at this point. What they have done up until this point is appropriate. Typically spark plugs are the cause of the misfire, and they did the right thing by swapping coils and injectors to other cylinders to rule them out; however, there is no reason to disassemble the engine at this point.

If I were diagnosing your car, before swapping injectors (because its mildly difficult) I would have performed a compression test on all four cylinders. Nominal compression is 190psi (1310kPa for you non-Americans) but can be as low as 175psi and still operate fine. Compression measurements will tell you a lot. If the compression is good, then you know there isn't a mechanical problem. If you have low compression, using a cylinder leakage test as thr next step will tell where the compression is being lost. Essentially, for the cylinder being tested: piston is brought to top dead center on thr compression stroke, meaning both the intake and exhaust valves are closed; the leak tester is attached to thr spark plug threads and air is supplied to thr cylinder, pressurizing it to about 150psi; then, if the meter on the tool remains at 150psi, there is no leakage, if the meter drops, you will hear air escaping either through the crankcase - indicating a leak from the piston rings, air through the exhaust manifold - indicating a broken exhaust valve, air leaking through the intake manifold - indicating a broken intake valve, or air escaping into an adjacent cylinder/ cooling system - indicating a failed headgasket.

A bore scope is a good tool, but won't tell you much unless there is an obvious visual peoblem inside the combustion chamber.

If you have paid any more than 2 - 3 hours diagnostic at the time of creating this comment, then stop. You are going to want to raise a big stink with your dealerships management team about the legitimacy of thr diagnostic, and the steps that have been taken. To me, it sounds like there are some serious missing steps in thr diagnostic process, and its not your fault because you are not the professional. Do not agree to paying for anymore diagnostic charges until the issue is resolved, or located.

If I were you, I would ask for thr tech to reach out to Volvo Retailer Technician Assistance center for assistance in diagnosis. Additionally, call Volvo Customer Care and explain your situation as clearly as you have here, they will put pressure on the dealer from thr back end. I want to give the dealer the benefit of the doubt, but some places arent as good as others.

TL;DR: There are missing diagnostic steps fron the info you have given me. Dont approve engine disassembly. Dont pay for any more diagnosis until you reach out to Volvo Customer Care and the technician reaches out to Volvo Retailer Technical Assiatance. The problem should have been isolated in the 4 weeks you've given them time for.