And yet the passage containing the parable of the prodigal son is prefaced in vers 3 with the words, "so he told them this parable", for the trio of lost and found stories. Stating "the following is a parable" is also a heavy-handed way to make the hearer reckon with the fictional background of the stories. Perhaps I should look for a better term than heavy-handed though.
Actually this is one of the bigger problems for me with Tobit, which I just happen to be reading at this moment (before I even stumbled on this thread). I enjoy reading apocrypha books, but I must admit, I don't see them as inspired. This does not mean they have no value or don't hold godly truths, which is probably why they seem to be found where canonical books are found. I love Wisdom of Solomon.
This explaination you give here, however, seems to sweep this issue under the rug then really trying to explain it. Of course with a mishap like this no one is going to consider Tobit "sacred history". It's hard to believe a book written under the inspiration of the Holy Spirit would contain such historical flaws to merely que the reader that the book is fictional. It is not needed and uncalled for. When Jesus was telling His parables He never queued His audience that His stories were fictional by adding in historical inaccuracies.
I will seek to explain a little more why the fictional elements of the story seem okay to me and do not exclude the book from being considered inspired scripture for me.
Before I do, I should reveal my a priori assumption concerning Tobit...
I am Baptist, and though as a Baptist I hold the two Baptist distinctives near and dear, namely, the priesthood of the believer and the autonomy of the local church. Nevertheless, I temper my outlook with a spirit of catholicity, (small "c"). God is revealing Himself in and through the history of His people. Therefore, on the subject of what is to be included in the Bible, I accept the ancient traditions of the orthodox communions. So I approach these books as scripture to start and proceed to interpret from there.
So Tobit presents us with quite a number of fictional elements. How do I deal with that? Well, I have to broaden my previously parochial perspective and include that which I believe God has included. Am I committing myself to stretching any details to as long as I can make them fit? No. I am only saying I am committed to doing for Tobit that which we take for granted concerning the other difficult parts of the Bible like for instance the Book of Jonah. With Jonah, the apologetics and explanations which have been presented by Christians so that we can believe its contents, we now take for granted. To an outsider those explanations still no doubt seem like a stretch, but because we approach this subject with an eye of faith we see it as a believable story. I beleive Jonah was a real person, who prophesied in ancient Ninevah after living in the belly of some sort of large sea creature for three days. Do I undertsnd it all? No. Do I beleive it all? Yes.
With Tobit, I cannot believe the numerous anachromisms are "mishaps". I think even the common folk amongst the Jews of that day would have been able to point out some of them. For instance, Tobit is said to live in Northern Israel when the kingdom was divided in 921 BC; Tobias, Tobit's son, is said to be alive even after the fall of Ninevah in 612 BC, a span of time of 300 years. The names of Israel's nemeses, Nebuchadnezer, Shalmanser, etc. are mentioned but they are placed in the wrong sequence and impossible times and places.
I appreciate your questioning my hypothesis that the author was attempting to show the reader his work is not to be considered sacred history due to its being based in a fictional setting. Perhaps you are right and he did not need to do that in that way.
Perhaps, this was an ancient attempt at humor.
Humor is one of those things that does not translate well from one generation to the next, let alone across centuries, languages and cultures, so it would be difficult to proove this. But perhaps the numerous incongruities of history and place would have hit people in their funny-bone in ancient times. For instance, if I were to say that George Washington and George Bush were good friends who often dined together, modern readers might get a feeling for the kind of off-kilter world the Book of Tobit sets for its historical background. I think there are other examples of seeming humor in the book, such as Tobias and Sarah going to bed on their wedding night and falling fast asleep!
In short, I accept Tobit as inspired scripture and holy writ despite the fact it seems to introduce a totally new type of literary genre to the Bible, that of inspirational romance novel, though perhaps there is a better moniker for it.