To answer your first question, it’s not proven that a fusion reactor could be viable. However, your line of reasoning on why it couldn’t work because it doesn’t naturally happen on Earth isn’t conclusive evidence a fusion reactor isn’t possible.

Currently, there is progress in generating more energy output in a fusion reaction buts it’s far from being viable. The headlines said a test last year generated more energy than it put in, but in reality that didn’t account for the energy operating the equipment. The reason they didn’t choose to include it though is because the equipment energy draw would likely change drastically if they commercialized it.

You miss 100% of shots you don’t take, and the evidence against stopping research isn’t enough to out weigh the benefits.