There have been a few Fusion reactors built already. Thats not the issue. The issue is getting them to be commercial, viable and efficient. The ones that have been built already use more energy than they produce, just to keep the reaction going... Obviously not what you want in a commercial setting. But these experiments are critical. The scientists and engineers need to build on this "primative" work and from that make the breakthroughs to the holy grail! As for when? There is a running joke that is said when someone asks that question, viable nuclear fusion has been just a few decades away for a few decades. Its not going to be easy folks, but anyone who ever though it would be was foolish, I mean forget rocket science, brain surgery and all the rest... this is fusion.

As for disturbing the mass ratio of the earth and the moon, it would be the other way around for a start, the mineral yeild of mining helium-3 would be measured in kg. That is kgs of the isotope would be brought back to earth, while at the same time, hundreds of tons of equipment etc... would be transferred from earth to the moon to mine it. But either way the net gain/loss is so damn tiny compared to HUGE masses we are talking about, their would be no issue at all. Its like saying pouring teaspoons of water from the Atlantic to the Pacific is going to raise the Pacific Ocean's levels and flood that hemisphere... just not realistic.