Website of Senate Commerce Committee does list information for the hearing set for today (Feb. 15). The only witness listed is FAA Administrator (Acting) Mr. Billy Nolen.
The website listing appears to limit the agenda to the recent NOTAM system crash (and related technology or modernization items). As noted in the post itself, my previous posted information relies on Wall Street Journal reporting, with regard to other issues that will be raised.
As to the DoT Secretary not appearing, well, it's a political factor imo. The nomination for FAA Administrator has met some opposition, and as a loyal soldier in the president's cabinet, it appears very likely that the Secretary did not want to provide further proof of why aviation knowledge and experience matter so much for posts like FAA Administrator. "Mayor Pete", as he has been referred to, lacks such knowledge and experience. And opponents of the pending nominee cite his own lack of such knowledge and experience as their basis. So I could see the Secretary not wanting to testify and end up giving more reality to that basis of opposition. Of course, as SLF, I'm not much in a position to say, in effect, "if you can't fly it, your opinion is irrelevant."