Nihilus didn't blast a planet when he attacked Traya or the Exile, he only attacked them and not anyone else with a single, focused attack.
Impossible. Life draining powers are fueled by the energies of the consumed; in a sense, Nihilus can kill planets because planets possess a planet-sized supply of life. The idea that Nihilus would even attempt to use enough power to consume a planet so that he could gain the life force of one Jedi is absurd; he'd be putting far more in than he was taking out.
As for your second point; it's irrelevant. Ultimately, if twenty Jedi were actually combined into a single being, it wouldn't matter in the slightest how focused Valkorion's powers may or may not be; he stands no chance. Take a good look at Dorsk 81 at the time of his death; he essentially became the "collective" power of several Jedi and threw a fleet out of orbit, his body destroying itself under the strain. Or maybe it would be more down your alley to refer to Bane and the Brotherhood's similar ritual in PoD, which allowed Bane to effortlessly destroy an massive forest with lightning? You're overestimating the difference in power between relative Force users; even Anakin at his full potential, a being which would be greater than the Father, would only have been twice as powerful as Sidious. Killing a hundred Jedi and killing a being who has the collective power of a hundred Jedi aren't even close to being the same thing.
And we've seen Nihilus and Traya use it easily in combat situations.
Curiously, every time was against opponents that weren't even powerful enough to get near them.
In the only possible exception to this rule, Nihilus happened to have brought Sion with him. Odd.