Though I have traveled much in Switzerland, and occasionally to Germany, I don't recall ever using a Laenderticket, or reserving a seat. I most often travel in Switzerland with the HalfFare Card.

That said, I will be using Goslar (and later, Bacharach) as a home base, with daytrips.
I am having issues with comparing rail ticket prices with Laender tickets, when a day trip crosses the border into the next state.
Ex: Goslar (Niedersachsen)--> Wernigerode (Sachsen-Anhalt) 29 € for Niedersachsen ticket (2 persons) to which border town--Vienenburg? at 14 € per person RT additional OR to Ilsenburg ? at 6 € per person RT additional?
So, 41€, 43€ or Point to Point of 45.20 € for two?
OR,
Goslar--->Quedlinburg, etc, etc...my head is spinning!
Thankfully, our other target towns are in NS.
Likewise, from Bacharach, our day trips are all in the Rheinland-Palatinate.
And lastly, we will likely purchase a 2nd class Sparpreis ticket from Frankfurt Flughafen to Goslar on arrival, in order to use the ICE trains. We will likely have time to kill at the airport, but a later train involves only one change in Hildesheim, welcome after a sleepless overnight flight.
I have never purchased a train ticket in advance. Is a seat reservation mandatory? Advised? on this Frankfurt, direction: Berlin Ostbahnhof 2 hour segment? Are reserved seats somewhat easy to find?
Thanks in advance!

Any help much appreciated. Is there an easier way?

Well the Harz towns of former East Germany even give out a public transport ticket which covers their region. But since you are in Goslar ... well, the easiest thing is obviously to get a Quer durchs Land ticket for 52 Euro which makes you completely flexible. The Niedersachsen Ticket for 29 Euro ends in Vienenburg see here mdb_285764_niedersa_ticket_eltungsbereich_ab_091218.pdf but you'd have to pay 28 Euro for two persons to Wernigerode and back so that's not an option. The cheapest option if you only want to go to Wernigerode on one day is indeed buying a point to point ticket at 45.20. For Quedlinburg the Quer durchs Land is always the best and cheapest option.

You could ask for a Rail & Fly ticket which most airlines offer which gives you the flexibility to travel to goslar. If you want to buy a ticket in advance seat reservation is not mandatory. Advised? Well, depends on the time of the day you travel. and if your train departs from Frankfurt or if it starts in Stuttgart.

Thanks Lubitsch,
For some reason, I am finding rail transport in Deutschland to be more difficult to get a handle on than Switzerland. And more expensive! Is there anything similar to the Swiss Half Fare Card, available for short term and tourist-eligible?

From Frankfurt Flughafen, the ICE 1698 departs dir: Berlin at 12:09.
Where can I see if FRA or Stuttgart is the starting point? That IS a good point. If just starting at the Flughafen, we could be sure to get aboard and claim two seats more easily!

Well, Germany is quite a bit larger, there are more local transport groups (here dating back pre-reunification) and crossing the border between them can be inconvenient and expensive. As I said if you'd do an overnight in Wernigerode or Quedlinburg you'd get a HarzTourCard which allows you free public transport throughout the East Germany Harz administrative district. But apparently no one has connected the Harz as a whole transport wise, there is only a Harz card for reduced entry to many places. Technically having all regional transport in Germany for 26 per person in your case is not a bad deal, it's just some short term journeys which can be a bit expensive, I agree on that.
Getting a Bahncard 25 or 50 is unlikely to pay off for such a short term visit though you can do the math for yourself.
You just clock on the number of the ICE yourself in the more detailed description of your journey. ICE 1698 starts in Frankfurt Airport so a reservation strikes me rather as unnecessary even though sunday is a bit more travel intensive.

I rode those Harz trains long ago - they were steam trains then and I think now? And I assume you're going up on the Brocken Bahn mountain train to the summit?

Thanks again Lubitsch. At least you have confirmed to me that there is not an easier way, which I was missing!
We have rented a FeWo in Goslar, so I guess no Harzkarte.
If you are familiar with this area, could you comment on getting by train to the Brocken?
Or is it worth a nice little hike up ?from which starting point-Torfhaus? Train back down to Schierke? Vice versa?
I'll be starting another thread to tighten up our startup destinations...

That should be 'daytrip' destinations!