Ocktoberfest Trip: Dusseldorf --> Munich (12 days)

"---This is where I need some suggestions. Do I stay in Cologne 2 nights? Or on our way to Oberwesel (to stay in a castle!) I was thinking we could stay a day/night in Koblenz or Boppard? Thoughts?"

The attractive towns are roughly from Braubach south. Braubach is home to Marksburg Castle and a lovely town as well. You might stay there. Our you could tour Marksburg (the only never-destroyed castle tour on the Rhine) and continue south to the Filsen ferry and float across to Boppard for a night. Boppard is busier with more dining and lodging options. I would avoid Koblenz - very little charm there. You can easily find lodging and other info on Boppard - Braubach is a little less "discovered" and much smaller.

Braubach's old town (Zum Goldenen Schlüssel has good food AND offers rooms as well.)
Braubach lodging options (there aren't many Braubach listings on the usual booking engines.)

3 days in Frankfurt? That's a lot - if you aren't into lots of museums, anyway.

The "Romantic Road" is nowhere near Oberwesel. It begins essentially in Würzburg (northern Bavaria) and continues south to the Alps. Würzburg is a very nice place surrounded by other really nice places - Bamberg, Iphofen, Sommerhausen, Marktbreit, and maybe Rothenburg if you don't mind being a tourist ant. This area (known as Franconia) is a better stopover on the way to Munich than Frankfurt and Stuttgart combined, IMHO, both of which were very heavily bombed 70 years ago and are for the most part modern, mostly post-war creations.

Some tantalizing photos of the wine village Iphofen
Intro to Marktbreit and Sommerhausen

"can we pick up the Romantic Road from here to Oberwesel to Frankfurt?"

The Romantic Road runs north-south between Würzburg and Füssen, far to the east of Oberwesel and Frankfurt(Main).

Thank you, Russ! That is very helpful!

"should we chose to take our route to Stuttgart or should we travel down part of the the Romantic Road after Frankfurt on our way to Munich?"

I'm not a devotee of following the RR route especially. Check it out and see which towns appeal to you and maybe visit some of them. But you aren't required to follow the route. I am more impressed by Iphofen and Bamberg (its medieval old town is a UNESCO World heritage site) and Bad Windsheim (open-air museum there is like time travel) for example than the RR, overall, which IMO is somewhat overrated and can get crowded as well. Lee (he's a good Joe, really) and I were both impressed with Nördlingen, which lies further south on the RR - not overrun by tourists, lovely old town wall walk, a living-breathing German city, not just a tourist magnet.

I think you are missing the boat (or the train in this case) by not using trains for at least part of your trip. You mention a lot of big cities you are visiting where a car will be a burden rather than an advantage. Traffic and expensive parking fees are the problem. Of course rent a car for a few days to get out in the country, but the trains go city to city with ease.
Something special in Duesseldorf? Otherwise the train from D'dorf airport goes on to Cologne in 30 minutes.
Your interest in Oktoberfest shows that you may be interested in a good party. If so, take this opportunity to visit a Strassenweinfest. They have one the weekend of your arrival in Ediger-Eller, just south of Cochem on the Mosel river. http://www.mosellandtouristik.de/en/wine-cuisine/wine-events/wine-festivals/ev/20160909/Strassenweinfest_Ediger-Eller/deskline.html
These involve shutting down the center of town, and all the wine makers set up booths for wine tastings. Wandering brass bands go from booth to booth to play, attract customers, and get paid with a glass of wine for each member or the band. The bands from Holland are the best, since the Dutch have a rep for being the party-animals of Europe.
You could stay in Cochem (although if you could get a room at a Weingut in Ediger-Eller if you reserved now). Both are right on the Mosel Valley rail line, with a train every hour. Easy to get to on the train from Cologne with a single connection in Koblenz.
Rent a car afterward and continue on to Oberwesel. Again, a car is worse than useless in Frankfurt, or Stuttgart, or Munich.

Thank you so much, Everyone!!! Can someone tell me what the German Wine Route is exactly? Is that something worth doing?