Eurail Pass-can I use Germany-Only pass to take train to Strasbourg?

No, you can use it until you get to the German border, then you will need to purchase a ticket to get you from the border to Strasbourg. You can purchase the short portion that goes to Strasbourg ahead of time (it's a regional train and the cost is minimal) and just hang onto it with the rail pass.

Keep in mind that to get the most bang for your buck, you should take the train from Bad Säckingen that travels through Germany to Offenburg, as opposed to the train that travels mostly through France. Once in Offenburg you will transfer to a train that will take you to Strasbourg. To get the ticket you need for your short portion in France, you can just purchase an inexpensive ticket that gets you from Kehl, the border town to Strasbourg.

When I traveled this route, I bought the Europass 24h Mini Pass. You can buy it from a ticket machine in Offenburg (where you change trains for Strasbourg) or you can get it online. Scroll down to the very last ticket offer: https://www.cts-strasbourg.eu/en/online-store/fares/tickets (you might have to click the arrow next to Departement, regional and cross-border tickets to open it up). Make sure you buy the "family ticket" since there are two of you.

Once you transfer in Offenburg, the train you ride to Kehl (the border town) will continue on to Strasbourg, so do NOT get off the train in Kehl. Just stay on board until you arrive in Strasbourg. And make sure you have your rail pass and your ticket from Kehl to Strasbourg handy for the conductor as needed.

ETA: I forgot to mention that the 24hr Mini Pass will allow you to use public transportation in greater Strasbourg for 24 hours.

Bad Saeckingen is on the Swiss border and the fastest way is to go to Basel, then to Strasbourg on a French regional train. However, like you say, you want stay in Germany as much as possible. When you use the Bahn journey planner, put in Offenburg as a stopover. You will still get routed into Basel, but to Basel Badischer Bahnhof, which is considered a German station and valid with a German Railpass for trains going to and from Germany, then a train to Offenburg.

Thank you so much! Mardee's in particular sounded like Gold to me. Question is: We are at the point, wth all the trips we are taking, that we think we will be better off with the Germany-Only Railpass; since we will be in Strasbourg 3 days, I guess it depends on how much the 'single day' costs for the 'short trip' to Strasbourg from.. Bad Saeckingen (with the change in Offenburg).
Perhaps you can advise also on this: Should we be reserving now,for our early October trips from Kassel to Berlin, and then from Berlin to Munich?
My sister (far away from me) is doing all the heavy duty planning due to my work schedule, but I am the Question asker.
PS for all: we are leaving from Bad Saeckingen because we're going for 3 days to Willaringen, to Haus Heidi Hottinger- not me, just a very very distant relative and friend who I've been pen pals with for about 20 years, and who we stayed with on a previous trip. She runs a most gloriously appointed GastHaus which will be the lowest price we pay for any of our accommodations on this trip, and it even includes picking you up at the train station!

The fare from Kehl (last stop in Germany before entering France coming from Offenburg) to Strasbourg is only 4.60 EUR.

Seat reservations are cheap, 4.90 EUR per person per trip, so go ahead if you are concerned. At least that way you are sure to sit together.

I'm staying in Strasbourg and planning to walk across the pedestrian bridge to Kehl for no other reason than I think it will be cool to walk from one country to another. Depending on how I feel that day, I can either take the tram to the nearest exit in Strasbourg then cross the bridge or walk from where I'm staying (a little over an hour), so your idea of using only the trains in Germany is quite doable.

Perhaps you can advise also on this: Should we be reserving now,for our early October trips from Kassel to Berlin, and then from Berlin to Munich?

If you know the dates, it's not a bad idea. Those routes won't be terribly crowded in early October, but as Sam pointed out, this way you will have a choice of seats and can sit together. Most trains have the option of tables, too, so if you want to sit at a table, it wouldn't hurt to reserve your seats early.

Thank you very much, everyone!