I'm doing a project on something similar to this, so while I'm not Gavriel Rosenfeld, I can give you three pieces of information that might shed some light on the publics perception of Hitlers, and his rhetorics, seriousness. I'm going to use "Hitler" to represent the sometimes complex relationship between the Nazi party, Hitler himself and the propaganda at the time. I have myself interviewed a Hitler Jugend- and a holocaust survivor, but I haven't published those interviews, so I'll abstain from using them.

There were extensive work done by Hitler to

• normalize the hatred. [1]
• have a "larger than life" persona, so there would be doubt if he was serious or not. Resulting in many underestimated Hitler's political skills and ideological resolve. [2]
• simply lie about the worst things. [3]

So to the question "was there doubt of Hitlers ideological resolve, during his rise to power"? Yes.

[1] https://www.smithsonianmag.com/history/how-nazi-normalized-anti-semitism-appealing-children-180959539/

[2] http://etheses.lse.ac.uk/1459/1/U105259.pdf

[3] Chapter 10 of James Murphy's translation of Mein Kampf