What your saying about reading the bible in context makes perfect sense for most books.
But it brings up some problems for Christians. If you start picking which parts apply to you and which don't, you can pretty much read anything you want out of the bible.
Worse yet, it greatly weakens the impact and power of the bible because it now depends on highly falible human judgement to mean anything.
In any case it's very hard to read the sermon on the mound as only having been given to the inner circle disciples. The end of the speech makes it clear that it was given to a large crowd.
In most bibles it's not clear from the first few sentences who it's being addresed to. Young's Literal is the only one I saw that addressed it to the masses, the one you quote below is the only one I've seen that aims it at the disciples specifically. The rest just address it to 'them' without being clear who they are.
Of course, that we can even have this argument show another huge flaw in using the bible as a source of Gods word.
Jay