To your last question, more like see no evil, hear no evil, speak no evil.

FDR's long time physician VADM Ross McIntire gets deservedly blamed an awful lot for keeping the lid on the disaster that was FDR's health in 1944 and 1945, but there's plenty of evidence that the patient himself both didn't want to know and didn't want anyone else to know as well. As FDR began deteriorating, McIntire brought in several specialists, including in March 1944 when cardiologist Howard Bruenn was just appalled with the results of his examination. Despite the dreadful misunderstanding of hypertension during that era where it was considered normal for blood pressure to rise with age (which allowed for one 170/88 reading of FDR's to be considered acceptable rather than what it'd indicate today, which would likely be an intervention involving hospitalization), he peaked at 240/130, which was terrifying even to physicians of the time.

But a significant part of the problem was that the President didn't want to discuss his health with anyone, and being FDR, in turn nobody could bring it up to him either. His niece Eleanor said he never used the word sick, although he used 'tired' a lot and would do things like cancel lunches, spent a lot of time recovering at Hyde Park and Warm Springs over that last 16 months, and routinely started sleeping in to the point where his first meeting of the day - always with chief of staff FADM Bill Lahey - would routinely be pushed back until noon or later, along with his visits to his beloved Map Room for communications and observation dropping off as well. George Elsey notes that Lahey and naval aide RADM Wilson Brown would often sit in the hallway outside the Map Room for long periods of time just waiting for FDR to finally come down the elevator from the residence without ever quite knowing when he'd show up.

To give an idea of just how little FDR wanted to admit to himself that he was in trouble, what's jawdropping is that despite all of Bruenn's work (he dropped most of the rest of his practice to attend FDR, ended up going to Yalta as part of the entourage, and actually was the physician in Warm Springs that initially attempted artificial respiration and called the death), his cardiologist never got a single question about what his examination had turned up or the medication he prescribed - and never felt that he could bring it up on his own. Maybe McIntire with his decades long relationship or FDR's daughter Anna could have (his relationship with Eleanor had gotten very distant again by 1944), but that really goes to the heart of the problem: FDR ignored his health and as the Boss, everyone else had to ignore it too - at least to his face. This isn't to say actions weren't taken; besides the single medication available at the time (digitalis) and diet and workload modification, even as early as 1943 FDR's plane rides were altitude limited to try to keep his SpO2 up, something that's important in patients with congestive heart failure, one of the several conditions Bruenn diagnosed.

In terms of successors, it's also worth going back to 1940 and noting that FDR had made significant moves towards trying to get Harry Hopkins to succeed him so he could retire to Hyde Park, but between Hopkins' Crohn's/stomach cancer/celiac/whatever (the negligent disaster that was Hopkins' medical care makes FDR's look mild in comparison) along with the significant enemies he'd made both on and off Capitol Hill, that attempt got nipped in the bud. FDR felt there was nobody else capable of leading the country through the cataclysm he saw on the horizon and felt he had no choice but to run again in 1940, and by 1944 Hopkins was even sicker and had made more enemies - including falling out of favor with FDR despite still being worked to death (at times almost literally) by him.

But as FDR's deterioration became more and more visible, the party bosses like Bob Hannigan knew that something was very wrong and knew they had to take action with the next Vice President since there was a good chance one way or another he'd be in a different job before the end of the term. How did they choose? As discussed, Hopkins was out. The various general officers were very much in the midst of fighting the war so were never even considered. Other party stars in the 50s like Stevenson and Kefauver hadn't really made their marks yet. That left only a handful of others.

Jimmy Byrnes had proven himself to be a decent administrator and in a pinch could probably do a decent job as President, hence why Truman chose him as his Secretary of State - which under the succession rules at the time was next in line when no vice president was in office - as soon as was logistically possible, partially as a safety measure for continuity of government. As a segregationist South Carolinian, though, he was considered probably unelectable and would have alienated labor and liberals and dragged on downballot races everywhere but the South. Worse yet, the arrogant and ambitious Byrnes took the 'assistant president' moniker the press bestowed upon him seriously and all involved - including FDR - felt he'd try to co-opt the stage from whoever was President until he had the office. To his eternal chagrin, Truman found this out the hard way multiple times throughout 1946 and 1947. There's pretty good evidence FDR took a couple of rather cold blooded steps to pull the rug out from under Byrnes' potential campaign from someone he'd been friends with for decades and needed to perform jobs that were demanding, essential, and generally unrewarding.

Wallace? Terrible judgment, which FDR had already concluded and was a major factor in not retaining him on the ticket; this played out when as Secretary of Commerce under Truman he urged sharing of nuclear secrets with the Soviet Union even as that relationship had already deteriorated and then capped it off with an isolationist barnburner of a speech that finally got him fired in 1946. William O. Douglas? Charismatic, brilliant, solid liberal credentials (and as a great poker player and story teller, FDR most enjoyed his company out of all the potential VP nominees), but it was entirely unproven that he could campaign and get elected to anything. Furthermore, while he was a WWI veteran, his only executive experience was as SEC Chairman in the 1930s and had little idea of what running the war effort actually entailed, although a rather ambiguous note from FDR elevated his candidacy.

That left one Harry S Truman. He was electable as a moderate who was popular among Southerners and at least then was acceptable to labor and not anathema to liberals. Unlike in 1940, by 1944 his work and ethics displayed on the Truman Committee (Marshall had told him in a rare moment of praise that it was worth a couple of divisions) was respected by even Republicans - despite some grumbling outside Capitol Hill that he was still a failed haberdasher and farmer and came out of the Pendergast machine. And last but not least, even without the organization of his now-jailed patron or the national party in 1940, he had pulled off unlikely odds to keep his Senate seat and was a hell of a campaigner.

While we don't have exact records of what was said, all this was all discussed in a meeting at the White House a few weeks before the convention between the bosses and FDR, and a strategy was set. FDR didn't make a firm decision but did instruct the bosses to narrow down the candidates with his suggestions and manage the process at the convention (Byrnes was initially popular, and there was a late Wallace surge.) Once Truman arrived to Chicago to give a nomination speech for Jimmy Byrnes as he'd planned, though, he got a surprise. He was hauled into a hotel suite with Hannigan and the other bosses, and there, with a prearranged and pre-scripted phone call with FDR bellowing into the phone loudly enough so that Truman could hear him clearly through the handset held up by Hannigan several feet away, he was referred to as 'that stubborn fellow from Missouri...who would break up the Democratic party in the middle of the war!' if he didn't accept the nomination. He did without further protest, despite Bess being absolutely furious at him for doing so - I can't find it in a brief search online, but there's a picture of her glaring at the convention that is genuinely hilarious.

So, even if he'd clearly decided that whatever energy he had left was going to be dedicated to winning the war and setting up the post war world rather than delving fully into intra-party politics as he had before, FDR was still involved. Even with his significantly reduced role, given how history turned out it was still one of his best decisions.

See: American Experience FDR/Truman (for televised interviews with principals, including Hannigan's daughter and remarkably, the nonagenarian Bruenn), Herman, Jan, Oral History with Dr. Howard Bruenn, January 31, 1990, BUMED Oral History Collection, McCollough, Truman, O'Brien The Second Most Powerful Man in the World, Smith, FDR

Edit: cleaned up and expanded a little.