Reply #2: Suppose the first lecture raises conjectures such as this:

[Boojatta]

Perhaps Mozart died of chronic kidney disease.

You could tell the audience that we don't know for sure whether or not that is true. Then you could tell the audience that we do know a particular point of Islamic dogma. Continue in this manner. This will not make Islamic dogma appear ridiculous.

If the second lecture proceeds as described in the Original Post, then we can ask what difference between the two lectures explains why the second lecture makes Islamic dogma appear ridiculous. The answer is that the conjectures are too improbable. To say that they are too improbable is to say that, in a lecture that includes both historical conjectures and points of Islamic dogma, there are some imagined historical events that one is not permitted to describe as possibilities. Then one can ask: "which imagined historical events are too improbable?"