Let me summarize your scenario, because there is another contentious issue in this thread. That issue in general is the value of an invention to society as a whole vs. compensation (even development money) to the inventor. Few people have thought about the range of possible and reasonable outcomes.
In your scenario:
What is the "just" outcome? Will the inventor be forced to license his invention? Will Big Gamma be forced to cease practice of the invention? Well, to use the classic answer, "it depends." The general rule is that Big Gamma, if it wilfully infringes, is liable for treble damages; and even if willfulness can't be proved, infringement still makes it liable for damages to the inventor. The general rule is also that the inventor can force Big Gamma to cease practicing the invention. The cost to the infringer can be enormous. Kodak had to dismantle a production facility, remove many cameras and much film from the stream of commerce, and pay damages to Polaroid in an patent dispute that related to "instant" photography. Polaroid had no obligation to license a competitor, Kodak, and the notion of "adverse royalty" did not play.
But what if the invention has health consequences? What if "cold fusion" was demonstrated to have the ability to make unlimited clean power, eliminate all combustion related air pollution (cars, electricity generation, etc.), and do so without creating any harmful waste product? Would the courts force the inventor to license Big Gamma? If so, would the courts set the compensation due to the inventor? Tough questions, and I won't predict an outcome on the hypothetical. But, courts have permitted infringement, when the absence of infringement was shown to create a serious detriment to public health. City of Milwaukee v. Activated Sludge, 69 F.2d 577, 593 (7th Cir. 1934) (Court of Appeals upheld validity and infringement findings of district court but lifted injunction against the City of Milwaukee that would have closed a sewage treatment plant and led to dumping raw sewage of the city into Lake Michigan).
The balance between public health and patents is a contentious and hotly debated issue. Some countries do not permit patents on health-related inventions, such as ethical drugs (not my field, and I may get the countries wrong, but I believe India and Brazil fit this description). In other countries, devices that permit new surgical procedures (or non-invasive treatments) are patentable. Where to draw the line on patentable subject matter is a subject that is worthy of debate. Maybe, if cold fusion is such a public boon, we should NOT permit it to be patented at all. Whoa, that notion stirs the memory of our "how not to encourage innovation" discussion!
For cold fusion, another point to keep in mind is that scientific principles are not patentable. If cold fusion exists, the fact that it exists can not be patented. The inventor has to embody the reaction in something; a series of steps, a device, something. One can patent the furnace, but not the fire.
As for the present complaint of the "cold fusion" advocates (which I take as a call for more money) -- researchers compete for public and private money all the time. Human nature is that people are greedy, and when an invention has commercial value, it WILL be developed. The patent office has no ability to stifle commercial development, and it is not their business to encourage commercial development. The function of the PTO is to examine patents, and to grant them if the statutory requirements for patentability have been met.
People who "peddle" 200 MPG carburetors, free energy transformers, and cold fusion have some things in common: they are asking for money, and they won't (or can't) teach you how to make a commercially valuable one for yourself. Let me tell you, I have enough skill to make a carburetor. Just give me the drawings, and I will make it. I will sell copies at the local shop. If it works, people will buy them because it will save them money. The notion that big oil killed this is nuts. By their nature, patents MUST be public documents, or else people can't know they are infringing. So what if big oil "buys the patent?" Even absent any patent, an inventor is free to dedicate the invention to the public. Print the plans on your web site. If it's a patent, the PTO has already done this for you. Scammers don't, because it kills their scam. But what if big oil "buys the patent" and decides not to commercialize it? If the invention was commercially valuable and the patent taught how to practice the invention, big oil would have infringers. If there was a 200 MPG carburetor, there would be thousands of infringers. Big oil sues me, I say, "okay, I'll stop." End of issue (for me). No court. No trial. A million people do it? A million lawsuits? I don't think so. Kind of like enforcing that swing patent. What is the inventor going to do, scream at kids on the playground? Hahahahahahahaha!!
One of the products was guaranteed to give you more miles per gallon, and it sold for about $6/pint. There was a bunch of anecdotal reports that buyers actually got those improvements. There were a few red faces on the investors when I reported that the product was just kerosene. True, it has more BTUs/unit volume than gasoline, but that didn't account for the improvements in the anecdotal reports. I asked if they would change their driving habits in small ways like coasting up to red lights, avoiding rapid acceleration, etc., if they had just paid $48 per gallon of a miracle additive? Psychology at work! Pay money to get more MPG, and your driving habits will automatically improve! The seller was clever. It only claimed that it would improve the MPG, and that individual results would vary. Technically, that wasn't false since the heat content of kerosene is greater than that of gasoline. But the product was sold on hype, and the promise of something for next to nothing. Whether P.T. Barnum said it or not, "there's a sucker born every minute."