I want to thank all of you for the information and suggestions you have sent. I should have waited until this week to ask, because a few days after I wrote, we left on a camping trip at the Oregon Coast where we didn't have access to email, phone service, etc. We're back home now, and I'm trying to get caught up on things, and work on our travel plans.
At this point, we plan to travel from London to Amsterdam by train (through the "chunnel") on Sept 29 or 30 - our daughters will be flying out of Amsterdam on the 30th, I think. We'll be on our own for the Netherlands and Germany. We want several days in Amsterdam, but those can be either before going to Groningen and Ostfriesland or after we return to Amsterdam. We're flying home from Amsterdam because we can fly direct to Portland now. (We haven't bought plane tickets yet, so the return date is flexible.) And we will probably rent a car in Groningen to travel in that area and into Ostfriesland so we'll have more freedom to visit rural locations and other towns.
I've printed out Individual pages of all of my husband's ancestors who lived or died in Ostfriesland or the Netherlands, which should make it easier for me to look up information here, and during our travels, and also gives me a place to make notes for any information I find. I also read through my travel journal from when we traveling in the Netherlands in 1987, and found that my husband's Mulder GG Grandparents were buried in Zuiderbegraafplaats (literally, South Grave Place?) in Groningen. In 1987 we took photos of the memorials, but sadly, I haven't been able to find the hundred or so slides that I took on that trip... (I haven't seen them for years. I guess that's one of the problems with living in one house for almost a half century - things can simply disappear!)
Thanks again, and I'll be in touch with everyone who wrote. (Several emailed me directly, too.)
Bonnie (Strong) Bronleewe