Your description definitely points toward; tire, wheel or balance problems.
The first thing you need to do is to ENSURE that they are doing a DYNAMIC balance. This requires correction weights on the inboard AND the outboard lips of the wheel. It is EXTREMELY common for the kids running the wheel balancer to STATIC balance with weights ONLY on the inboard lip of the wheel. They do this to keep from scratching the outboard portion of the wheel, probably because they scratched one and got in trouble.
YOU CANNOT PROPERLY BALANCE A WHEEL WITH STATIC BALANCE. This worked many years ago with solid axles and big bulky suspension systems. MB's and almost all modern cars have very nimble suspension systems through which such vibration easily travels.
By not balancing properly with weights inboard AND outboard, the wheel basically "wobbles" causing vibration at higher speeds.
If all these rebalances have indeed been done with weights inboard AND outboard, it is time for you to seek out a shop with a Hunter GSP9700 wheel balancer. You can go to www.gsp9700.com and find a search feature that will tell you what dealer or shop near you has one of these machines. This machine with a competent operator is capable of telling you if the problem is a bent wheel, a bad tire, or can even tell the operator where to rotate the tire on the wheel to correct the problem.
Before you bother finding a 9700, simply walk outside and see if you have any wheel weights on the outboard lip of the wheel. Also the outboard correcting weight can be accomplished by using a tape weight on the inside diameter of the wheel, toward the outboard edge. A shop that cares about balancing correctly will be aware of how to accomplish this with tape weights.
The necessary ingredients for a proper wheel balance is;
20% Any computer balancer in proper working order
1% Operator minimum IQ of 65
79% An operator attitude of wanting to do it right
The rest is, of course, non defective tires. This is the case 999 times out of 1,000. A bad tire, that absolutely cannot be balanced is rare. If the tire is Michelin, Continental or Yokohama, it is very rare due to different tire construction for these brands. There are a few other brands that use "pieces of pie" molding rather than a clamshell as were used for years by everyone.