Is it bad form to use the word “novel” when describing your own work in a paper?
Yes, I would advise against using the words novel or new for the following reasons (which overlap with what you already argued):


It’s almost impossible to fully ascertain that something is new. Even if you just discovered some outstanding and surprising effect like superconductivity, somebody else may have discovered it a month earlier and is just about to publish it.
At the same time, you are expected to have ascertained the novelty of what you are doing as well as reasonably possible and to document it in your introduction or literature review. To quote JeffE: 


  The message you want to send is "I have read every paper on this topic, and none of them do this thing that I'm about to do."


If you have any reason to believe that what you are doing is not novel, you should either document this (e.g., “we tried to reproduce this experiment”) or not try to publish at all. Thus, there is never any reason to claim something is novel – it is implicit that everything you write that is neither common knowledge nor equipped with a citation is novel to your best knowledge.


The following memo by the American Physical Society (one of the most prolific publishers in physics) argues similarly:


  Physical Review adheres to the following policy with respect to use of terms such as "new" or "novel:" All material accepted for publication in the Physical Review is expected to contain new results in physics. Phrases such as "new," "for the first time," etc., therefore should normally be unnecessary; they are not in keeping with the journal's scientific style. Furthermore, such phrases could be construed as claims of priority, which the editors cannot assess and hence must rule out.