[Morwen]

I don't agree that Gandalf is "overly concerned" or obsessed with the Shire. Gandalf is constantly on the move, he is Gondor (though perhaps not as welcome there after Ecthelion's death), in Rohan, searching the Mines of Moria for Thrain, in Mirkwood helping to defeat the Necromancer, etc. He certainly visits the Shire and it is said that he was a friend of the Old Took but when Bilbo meets Gandalf in An Unexpected Party (TH) he fails to recognise him as he had not visited the Shire since those of Bilbo's generation were small hobbit boys and girls. This doesn't exactly show an obsession of with the Shire. As for the visits that he paid to Bilbo afterwards - (a) Bilbo, like the Old Took before him, was Gandalf's friend and (b) he was concerned about Ring that Bilbo found and its effect on Bilbo because of Bilbo's initial lies, his anger when advised not to use the Ring, the fact that he showed no signs of aging and later that Bilbo felt that the Ring was growing on his mind.

After Bilbo's disappearance Gandalf says to Frodo,

"I may be away for a good while; but I'll come and see you again as soon as I can. Expect me when you see me! I shall slip in quietly. I shan't often be visiting the Shire openly again. I find that I have become rather unpopular. They say I am a nuisance and a disturber of the peace. Some people are actually accusing me or spiriting Bilbo away, or worse." - FotR, A Long-Expected Party

Of the visits that Gandalf does pay to Shire after this, the following is said:

It was just at this time that Gandalf reappeared after his long absence. For three years after the Party he had been away. Then he paid Frodo a brief visit, and after taking a good look at him he went off again. During the next year or two he had turned up fairly often, coming unexpectedly after dusk, and going off without warning before sunrise. He would not discuss his own business and journeys, and seemed chiefly interested in small news about Frodo's health and doings. (my emphasis)

Then suddenly his visits had ceased. It was over nine years since Frodo had seen or heard of him, and he had begun to think that the wizard would never return and had given up all interest in hobbits. But that evening, as Sam was walking home and twilight was fading, there cam the once familiar tap on the study window.
FotR, The Shadow of the Past

His visits are because of his legitimate concern about the Ring that Frodo has inherited from Bilbo and the implications not just for Frodo but for all of Middle Earth. Since these visits are directly tied to his mission they can't be evidence of an increasing obsession with the Shire.

And I don't see how his smoking gets in the way of his mission. One can smoke on the go. There is no indication that Gandalf took to lazing around under large trees, puffing away without a care in the world.

Originally posted by Sardy

Now, my question for this hypothetical is this: Had the Ring not been found by Bilbo, and had the Necromancer waited another thousand or two years to re-emerge, might Gandalf have strayed from his mission? Become overly distracted with the Shire? Gone Native? Lost his way?

I don't know that Gandalf was straying from his mission before Bilbo found the Ring nor do I think that he was "overly distracted" with the Shire. He is friendly with certain hobbits who live there and visits them from time to time. But unlike Saruman who establishes himself in Isengard or Radagast who at one point was living near Mirkwood, Gandalf has no home base in the Shire. His mission entails moving around, going among the various people of Middle Earth and that is precisely what he does.