It is very likely. Just like it was in 2001 and 2005, and 2009. One party government. Democrats would be absolutely devastated, and you may see people like Kamala Harris, Cory Booker, Tim Ryan be more vocal leaders of the Democratic Party.
Why 2009?
But I do agree that Democrats are going to need to get people like Kamala Harris and Tim Ryan to be more prominent in the future. Hopefully not Booker, however.
At the beginning of the year, OH and FL were critical for a D majority. But now Democrats are focusing on IN, NH, PA, and MO, all of which are at least tossups for the Dems right now. (they are already well ahead in WI and IL) That would give them 52-48 if they win their best targets.
Yes, but wrestling MO away from the GOP will be tough for democrats. There's no Todd Akins running to save them this time. NH, PA, NV, and NC are what will determine the majority.
I mostly agree about with your characterization of Senate races, but I don't think that Trump winning would guarantee a GOP senate.