1) A blast that will kill 6 to 10 goblins would probably kill one big orc.
If he could do that, why doesn't he blast the worgs, or the trolls, or the wolves, or the Watcher in the Water? Why does he need to create a detraction of smoke and fire if he could just blast everything in his path with lightning? Read the Silmarillion. Not even an unrestrained Maia can do that.
2) Gandalf testing out his awesome new sword on The Great Goblin is not proof that he could not have killed the monster in any other way.
3)Nazgul are powerful sorcerers. If he were facing The Nine in Dol Guldur (perhaps a transplant of his battle with them that was omitted from the FOTR film) you wouldn't hear me complaining about misrepresentation of power. Are you suggesting Bolg is as mighty as The Nine?
Yes. Bolg is a first rank hero-orc, not cannon fodder. Bolg can beat Thorin. Thorin is a first rank hero, on a similar level to Aragorn. Aragorn can go up against Nazgul, so could Bolg (if they where on opposite sides).
As Gandalf The White, he was impervious to mortal weapons,
I see not indication of that in the books, and it seems unlikely, since Maia are killed by mortal weapons in the Silmarillion. The Balrog killed by Glorfindel for one.
but that is neither here nor there as we are discussing The Grey. Did I say he was invulnerable in his manlike incarnation? Don't recall having done so.
If he isn't invulnerable (which we know he isn't because he has his arm in a sling after the battle), then he can be defeated by an orc with a sword.
He does have superhuman strength. Not on the level of Superman or He-Man of course, but the books describe several situations where his hidden physical strength was revealed.
Wrong. it is clear in the Cahadras incident that Boromir and Aragorn are physically stronger than Gandalf, since they clear the path through the snow. And Legolas is faster.
his lifting Faramir from the pyre as easily as one might lift a small child, "revealing the strength that lay hidden within him. . ."
Lifting a single human if far from superhuman. Boromir could have done that easily. It's "hidden" strength because Gandalf looks like an old man.
He probably can turn invisible. Recall, after the flash amidst The Goblins, no one sees him again until he performs his fire enchantments in the cave of The Great Goblin.
If you can turn invisible, you don't need destractions of flashes, smoke, and flares to blind your enemies. You would be able to pass through Moria or Dol Guldur in total safety. And it makes magic rings wholly unremarkable. There is no indication that Sauron or any other Maia can become invisible without the aid of Magic rings (Sauron could shapeshift though, and there is some indication that Radagast can too - THIS is a Maia power, but not one Gandalf ever shows any sign of possessing).
There are other examples of Istari limitations too. Saruman uses black powder to blast Helm's Deep, not magic (as he does in the animated version). He isn't able to blast the ents out of existence with fireballs. Indeed, if his magic was as powerful as you imagine, why would machines and technology hold any interest for him, when he could do anything more easily with magic?
That's quite enough. I'm sorry, but your ideas about what wizards can do owes more to Dungeons and Dragons (and the sort of fantasies that Sam Gamgee believes) that the subtle magic of Tolkein.