You say engine mount "bushing" - there is really only one of those - the "dog bone" shaped upper mount connected to the strut tower; the front and rear engine mounts are fluid filled - not elastic bushing type. You can easily inspect the upper mount with a flashlight for cracks or deformation. If it is sound, the others are likely OK. I suspect that once this "engine moving control rod" as Toyota calls it, goes, it places additional unintended stress on the others. You are more sensitive to the correct operation of your vehicle than the Dealer service advisors. I remember the mocking look I received when describing a faint odor of burning rubber. The engine compartment looked immaculate, and as in your experience found nothing wrong. Some months later, when checking the resistance of the spark plug wires with a digital multimeter, and finding two of the four with infinite resistance, I discovered one of the boots shorting to ground deep inside the spark plug tube - hence the burning rubber odor. Don't know if it makes a difference, but I have the California version with the external coil. The spark was still strong enough that missing was only slightly noticeable under harder acceleration. Being sensitive to slight changes is a good thing. Yesterday morning, while checking oil level, I noticed a very slight amount of seepage at the valve cover gasket which I replaced apx. 15k miles ago. Checking the torque of the 30mm mounting nuts found them to be very loose and re torqued them to spec. If your engine is smooth at idle, I'd check the spark plugs, wires, coil, and cap and rotor if you have them first for a slight miss.