The key answer on the diner, dealing with a major theme of the movie, comes in the form of the quote of the old farmer (or rancher) who the police question at the diner:
"Those men robbed the bank that's been robbing me for 30 years."
-Paraphrased from the movie
These people hold some admiration for the bandits, and certainly aren't going to sell them out to the bank that's been bleeding them dry for decades. They aren't bound by the letter of the law, but by their personal codes of ethics.
It's helpful to know the history of
"bandits rob with a gun, bankers rob with a pen"
featured in Woody Guthrie's "Pretty Boy Floyd", a song about a depression-era bandit hero. During the Great Depression, people couldn't pay their mortgages and were oppressed by the banks. Outlaws such as John Dillinger and Bonnie and Clyde were folk heroes. Hell or High Water is set in the wake of the Great Recession caused by a real-estate collapse based in large part on predatory loans. This directly relates to Toby's need of cash to pay off the predatory reverse-mortgage the bank tricked his mother into signing so they could effectively steal her land.
The waitress's refusal isn't inexplicable either. She's attracted to Toby, and even though he gently rebuffs her, he still tipped her $$$. By contrast, the cops are the guys taking that $$$ that she needs to take care of her kids. This violates her sense of justice to the degree where she berates the cops. Her determination of who are the good guys and the bad guys is not based on the laws of the state, but on her own moral compass.
On number 2; maybe, but it would be hard to convict based just on thinking it was Toby's voice. There was nothing about them that particularly stood out, so the potential base of possible suspects would be very large.
On number 3; they specifically only rob money from the trays because it's unmarked. The banker receiving the money could be suspicious, but he can't prove anything.
Also, Toby was advised by his lawyer to open up the trust account at that bank to keep things as copacetic as possible. i.e. the greed of the banker/bank works in Toby's favor - they'd rather have the cash back in an account with their bank than get into a lawsuit with an unknown outcome. The bird in the hand over the possible two in the bush, so to speak.
Toby's lawyer is also operating under his personal code of ethics, taking on risk beyond his financial gains in service of what he considers justice, against the letter of the law.
On number 4; the assumption is that there is a reasonable amount of traffic. Texas is a massive state and you're talking about fanning out in all directions. Even at a 50 mile radius, that's a huge amount of territory to cover. Toby didn't look suspicious enough to be detained or searched further.
On number 5; I don't have an answer, but as careful as Toby was trying to be, what's to have stopped him from buying the car through a 3rd party or from some source that's not easily traceable? They were old cars after all and it's a rural area. Maybe it was unregistered.
Code of Ethics
But the film isn't concerned with the legal aspects. It isn't at heart a police procedural, but a Western, and Westerns are concerned with personal codes of honor. It's really about the conflict between Toby and Officer Hamilton's individual codes.
Hamilton is presented as a competent cop. If the evidence isn't there, we have to assume Toby covered his tracks well enough.
Tanner's sacrifice of himself also probably contributes to Toby's free status at the end. His death gives the Rangers a partial win, and Hamilton can't finger his accomplice.
Tanner's self-sacrifice also arises out of his own personal code. He doesn't explicitly state his reasons for choosing to die in a standoff, which is partly related to his self-identification as a modern day Comanche, defined in the film as "Enemy to Everyone", but it is presumed his motivation is also to protect his brother and nephews, gaining some degree of redemption for his wayward life.
As to Toby not getting caught; he is actually caught — it's just that it's outside the scope of the law.
In keeping with the theme, Hamilton doesn't care if he can convict Toby in a court of law. Hamilton's code calls for vengeance regardless.
Hamilton becoming himself an outlaw, just as Toby had been compelled to do, validates Toby's choice and is the final reinforcement of the idea that justice and the law are not always the same thing.
Where it becomes really complex is where sympathies lie at the end of the film. Although Hamilton is grieving for his slain partner, there remains the question of whether Toby can be held responsible for his brother's actions. From the standpoint of the viewer, Hamilton may be seen as the villain, exacting retribution on a good man for a killing he did not commit, robbing Toby's sons of their father.
Toby's stated willingness to accept the consequences of his action is a final validation of his innate morality. This idea is at least as old as the death of Socrates, and is commented on more recently by MLK in his Letter from a Birmingham Jail.