Steering wheel/gas pedal vibrations after new tires and wheels

I’m familiar with SE but new to the Mechanics side of the site.
Anyway, I bought new wheels and tires for my 2005 Mazda 3 S hatch back in June. I upgraded the then-current “MB” brand 16″ aftermarket wheels to “MB” 17″ aftermarkets (since the 3 is supposed to have 17s AFIAK). The standard tire size is P205/50R17 but I bought four Barum Bravirus 3s in P225/45R17 since they were relatively inexpensive and of decent quality (according to Discount Tire). I bought both wheels and tires from Discount Tire in one fell swoop during a sale. Since I upgraded I’ve had constant steering wheel wobble at speeds above 60mph. Before I upgraded, I never felt any wobble, and if there was any it was indiscernible. Here’s what I’ve done so far in attempting to solve the problem:

1. Took the car back to Discount twice for re-balancing. Apparently the tires were incorrectly balanced the first time and that did remedy the problem a little bit, but there is still steering wheel wobble. Discount insists that the wheels are not mis-cast and that the tires are just fine.
2. Took the car to my local, trusted mechanic for diagnosis. He replaced the left front CV axle, both front struts, and did a front-end alignment. He also said that the wheels didn’t have hub rings when they should. These changes improved the car’s overall ride but not the steering wheel wobble. Also, despite the alignment the car still pulls slightly to the right. (I can live with this.)
3. Back to Discount to get the hub rings installed, which did little if anything to remedy the problem. I even let one of the Discount employees drive it on the highway and he confirmed the wobble. He recommended I upgrade to a set of Yokohama YK740 GTXs in P205/50R17. I had them rotate the tires first to see if the problem might be with a particular tire or tires. The steering wobble is different now, because sometimes it vibrates over 60mph and sometimes it doesn’t. Now I’m getting consistent pulses through the gas pedal over 60mph. Have not taken the vehicle back to Discount to report this.
4. Per Discount’s request, I took the car to their recommended mechanic for a second opinion on what my mechanic did/said. He wants to replace both rear shocks and do another alignment.

I’ve already spent enough on this and I really don’t want to pay more to replace rear shocks. I don’t see how replacing the rear shocks would solve steering wheel wobble anyway. Any help would be greatly appreciated. Thanks!

Update: Upon doing some more highway driving, I’ve noticed that the steering wheel now wobbles at speeds as low as 50mph but doesn’t always wobble. (Sometimes it smooths out when I decelerate, like going from 65 to 60.) However, I feel near-persistent throbbing through the gas pedal. No problems driving under 45mph.