We seem to have reached an understanding of sorts.Theoretically the Earth could be ignited like the Sun, would the reaction stop when free or would more fuel be made available?
Fusion is occurring from hydrogen in the high pressures of containment, so you will have to add a little as it fuses into helium. Basically, keep feeding it hydrogen, and bleed off the product, and it runs for a long time.
And yes, containment failure would be catastrophic, but limited in nature. The temperature would rapidly collapse since fusion would cease on containment failure. Probably not as nasty as a nuke melt down, but very nasty in the immediate area of the 15 million K plasma venting!