It is not practical to test ignition coils without using expensive equipment. But your theory that more coils are failing is probably correct. These coils were not properly designed; and as a result are failing because voltage is arcing internally between the coil windings. There is one company, called Airtex/Wells, which has taken action to correct the coil design problem that is causing so many coil failures. When several coils fail, the others will be likely to also fail; so it is best to replace all the coils with parts that have been designed to last longer.
I would replace all the coils on this vehicle with a set of Airtex/Wells #5C1652. These parts sell for $76.79 each at www.rockauto.com. You won´t need to know the firing order if you replace all the coils; the length of the harness wiring makes it difficult to connect coils in the wrong firing order.