I'm willing to bet that Moe and Larry had an inkling that the jig was up, as it were, for short subjects, seeing as they were the only ones left in the department. Saying that, I'm not sure what they could've expected from Joe Besser, since they already knew of his style way before he was asked to join, and let's face it, to paraphrase Desmond, because of budgets, etc., Joe was IT.
I tend to believe his story about not wanting to tour with the Stooges because of his wife's illness--even though he did appear in
Say One For Me, it was filmed in LA, so it wouldn't have been much of a problem to get home at a pretty regular time, rather than touring for weeks or months on end.
As for Joe DeRita, we have 2 different stories, but since they're pretty much the same, I don't pay much attention to who's "right." Larry saw Joe at Minky's Follies, then told Moe that he'd be a great addition; Moe's version is that he asked Joe after Curly's stroke (although that's interesting that he said that on the Mike Douglas Show)--it doesn't mean that he couldn't be wrong, but it differs from what Joe himself said. If I recall, he stated that any rumors that he was asked to join before Shemp just added "romance" to the story.
Even though I've been one of the fans in the past to say that Curly-Joe (and Joe) weren't all that, reading more into the situation, I realize that it was pretty much inevitable that those 2 would be Stooges. In 1956, they needed someone to fill in
immediately, and who better than a fat guy who could (somewhat) match footage with the most popular fat guy? In 1958, who better to fill in quickly than a burlesque comic who could fit in faster than anyone they previously auditioned? As for feelings, Larry apparently felt rather strongly about Curly-Joe's contribution, threatening to quit if Moe didn't make Joe an equal partner. Moe apparently had no hard feelings, as he granted permission for DeRita (along with Mousie Garner and Frank Mitchell) to use the Stooge name for their combination. I'm not sure if this was sooner (I'm assuming it was, since the Howard/Sitka/DeRita pairing was mere months before Moe's death), but you wouldn't give permission for someone to use the name (for free!) if you didn't at least have a bit of feeling for them.
Interesting, though--did DeRita, etc., just ask Moe? Why not Larry (if this was before his massive strokes)?
Damn, I think I asked more questions than I answered!