As for Colonel Armstrong's first name, I think there are three explanations:
1. Colonel Arbuthnot deliberately gave the wrong name to give the impression he didn't really know Col. Armstrong very well.
2. A typo that was further messed up by a sloppy editor– since "R" and "T" are next to each other on the keyboard, it's certainly possible that Christie meant to write "Robby," but accidentally typed "Tobby" and didn't catch it, and the editor changed it to "Toby."
3. (The most likely in my opinion.) It's not that common, but "Toby" is sometimes used as a diminutive form of "Robert." It actually kind of makes sense, if you consider (I'm thinking of Hastings' musings in Peril at End House here) that "Peggy"" can be a nickname for "Margaret." Or, knowing how nicknames come about, perhaps someone once noticed that he bore a resemblance to a certain Toby jug and the nickname stuck.
But as you note, in Mrs. Hubbard's final comments, she refers to her late son-in-law as "John!" This could probably be written off as a mistake on Christie's part, although I am reminded of the big Sherlockian scholarly debate over Watson's middle name– we know his name is John H. Watson, but his wife calls him "James." People have discussed this for years, but Dorothy L. Sayers created the most widely accepted answer in her famous essay. Sir Arthur Conan Doyle only provided the initial "H.," but Sayers argued that since Watson had Scotch ancestry, his middle name was very likely "Hamish," which is the Scotch version of "James." Sayers argues that many wives found the name "John" too ordinary, and that Mrs. Mary Watson took to referring to her husband by the Anglicized version of his middle name. The middle name "Hamish" has been so widely accepted by Sherlockians that it's been incorporated into The Seven Percent Solution and in the BBC series Sherlock, Watson makes it clear in A Scandal in Belgravia that his middle name is "Hamish." I submit that something similar is at play here.
I therefore argue that the Colonel's full name was John Robert Armstrong, and that since "John" was a very common name, perhaps to distinguish himself from other boys, early in his life he started going by his middle name, "Robert," but was nicknamed "Toby" by his friends in the military. Agatha Christie Ltd. backs up this assumption with the case of Major (later Colonel) Despard. He's John Despard in Cards on the Table, but his wife Rhoda calls him "Hugh" in their second appearance in The Pale Horse. Agatha Christie Ltd. officially announced that his full name was "John Hugh Despard," and his wife addressed him by his middle name. It therefore makes perfect sense that Colonel Armstrong's name follows the same pattern.
Looking just at the one-hour episodes, which are your favorites? My favorites are "Clapham Cook" and "Veiled Lady" because of their humor, and "Plymouth Express" out of sentiment– it was the first episode of Poirot I ever saw.
My least favorite? "Missing Will," because it's so different from the original story.
Dr.Sheppard, you're right that Christie's name isn't as prominently mentioned as one would think it would be. Though Christie is mentioned lots of times in the various interviews and news articles, her name isn't on the teaser poster, although there is a "Based on the novel by Agatha Christie" line amongst the general credits of the teaser trailer. I'm not sure why "Agatha Christie" isn't featured more prominently.
I think that every writer who has contributed more than two episodes to the Poirot series has ups and downs, and many of them have made questionable adaptation decisions, but due to his pivotal work in defining the series and characterization, I believe I will give the nod to Exton.