This vehicle could have 2 separate problems and to be honest I don’t really know what to recommend as it’s difficult to determine what the shop has done as far as any diagnosis and how they arrived at their conclusion it’s a sticking valve.
The odds of it being a sticking valve is right around zero in my opinion and if they added a cleaner through the intake manifold in an attempt to cure a problem like this then it’s a very misguided attempt.
An off the cuff wild guess about the bucking above 45 MPH might be a clogged fuel filter.
The 300 code is a random misfire and could point to a crank sensor or even a chronic misfire due to a clogged fuel filter.
The cylinder 2 misfire, assuming spark plug and wire is good, may be related to an injector fault IF the compression on that cylinder is even half decent.
I see no reason to perform a leakdown test, which is essentially a more high tech and thorough version of a compression test.
A simple compression test will reveal what needs to be known in this case.