Dvar Torah/Speech before Musaf

At what point was the delivering of a dvar Torah, speech, etc., as we know them today, introduced into the davening (before Musaf) and what is the minhag in KAY regarding this? Does this have something to do with what used to take place in the time of the gemara? What is the appropriate seder according to minhag Ashkenaz? Is it after davening while people are still in shul? Was it added in the middle to ensure that people stayed (a captive audience, so to speak)? Is this inappropriate according to the original minhag and the davening should rather be one continuous stream? Was the original minhag to daven, go home, have a seudah, and then come back to shul to learn? Was this a way to ensure that people attended some sort of formalized shiur?

Steven