"Major regions rhine region inc trier / Chocem/ bernkastle and of course koblinz."
I assume that "we have a wedding in Koblenz" means "we" are the couple getting married. Congratulations!
So... is it also correct to assume that your wedding has you way "behind the curve" in terms of research and planning for this trip? Trier, Cochem and Bernkastel are a good distance up the Mosel River - not on the Rhine. See Rhine/Mosel map:
We can't fully plan an "itinerary" for you, but if you want a week in the Rhine/Mosel region after the wedding, I think your best bet is to start by getting reservations for 3-4 days in ONE Mosel town and again ONE Rhine town. I am going to suggest Cochem as your Mosel town, and St. Goar as your Rhine town. Cochem is half way between Koblenz and Trier (not a "dreamy hamlet" but a good place to visit on a day trip by train) and could also serve as a base town for easy day trips to Beilstein (by boat) and to Burg Eltz Castle (train + hike, or bus on Sat/Sun) and to Bernkastel (train + bus.) Like Cochem, the Rhine town of St. Goar is a very good base town for the Rhine "hamlets" of Bacharach, Oberwesel, and Boppard - all just a few minutes away by train. From St. Goar it's also easy to visit Braubach (Marksburg Castle) and Rüdesheim (famous wine town) on the opposite river bank by train (use the St. Goar ferry to cross to St. Goarshausen and board the train there.)
In St. Goar you can have a view like this one from your window or balcony at the Hotel Rheinfels or the Rheinhotel St. Goar.
For a nice cruise, take the train from St. Goar to Bingen and cruise back to St. Goar.
Your week's activities can be "planned" between now and the date of your stay - or you can just play it by ear once you arrive. It's very easy to catch a train from Koblenz to Cochem or to St. Goar - they run every hour. Local train tickets and day passes that cover train (and certain buses and ferries) are available at the station - you would never pre-purchase train tickets for these short distances.
(BTW... Marskburg Castle / Braubach are only a 10-minute train ride from Koblenz. So you might want to visit it from there instead of from St. Goar.)
It would be smart to pre-purchase a saver fare ticket for the journey from St. Goar to the Black Forest. Gengenbach is one good base town there.
G'bach participates in the KONUS program - you get a guest card for free train travel around the Black Forest from your innkeeper: