Eh, it's pretty different. You can join your powers in external telekinetic output, and in that area their power output together surpasses Skywalker. Obviously you can't join your output in terms of immobilization and personal resistance, but if you hypothetically mixed up Plagueis and Starkiller into one being, that being would resist the black hole with the Force too.
Well for example, Galen may lose to two Dooku's somehow combining their powers, but individually one Galen's lightning can <insert ridiculous TFU feat>...but two Dooku's standing together shooting their lightning couldn't, because that would just be doing twice as much as what Dooku can do, which wouldn't be enough.
But I agree that they could challenge Luke by the tactical factors of attacking him from multiple fronts or whatever.
I don't think so. If you look at the quote, the author calls him "the very essence of the immovable object" and then goes into great efforts to detail a bunch of increasing absurdities, as if to try to communicate to the reader how literal he means that without using the word "literally". It's probably what I would have done if I had wanted to say "this is literal" - use absolute statements like "the very essence of" and then talk up a bunch of absurdities. It sounds a little too much to be hyperbole.
Even if it were, tanking the telekinesis of Unuthul, who had the combined Force potential of trillions of Killiks, is pretty damn impressive.
What you would have done has no bearing on what the author themselves did and what their intent was. Personally I take that entire diatribe as hyperbolic as no force user has shown shields even close to being able to tank the matter disrupting energies of a black hole and because the quote itself was referring to Luke's immovability and not his force defenses.
Did he have their power or only their potential? Also how powerful are Krillik force users in comparison to a random sentient?