This is FASCINATING! I'm afraid I will pester you with questions until you're sick of me. If I get to be too much, please tell me to cool it.
So that flow chart is incorrect, because I don't see Kreis Alzey in it anywhere, right? How did you know that Oppenheim was wrong and how did you know to look at Alzey? No, "district" doesn't exist in the US, but I would compare it to "county". Incidentally, those Wikipedia articles were automatically translated into English for me, so no problem.
I tried using that wiki for another of my ancestral German locations, Obergimpern, but it didn't seem to recognize that. Wikipedia indicates that Obergimpern is a village that is part of the city municipality of Bad Rappenau (about 2 miles apart on a map), district Heilbronn, administrative region Stuttgart, and state Baden-Württemburg, at this time. However, my ancestor was born there in 1697, so my next task is to start going backwards in time. The wiki flow chart of Bad Rappenau doesn't seem to me to be useful.
Looking at Heilbronn, I see that it derives from Oberamt Heilbronn, formed in 1803, to which some lands were added in 1926, and even more in 1938 when it was recognized as a district. I have to stop for now. I'll look up the Oberamt, but I anticipate difficulty trying to find out if Obergimpern was part of the Oberamt or part of the lands that were added prior to the formation of the district of Heilbronn.
My hope is to walk through this process with you step by step so I can eventually learn how to figure out these place structures on my own. So more later.
Additional: It would seem that with the dissolution of the Holy Roman Empire, the lands including Obergimpern (WNW of Wimpfen) were added to the Margraviate of Baden in 1806 at which time the Grand Duchy of Baden was formed as the . Don't yet know where those lands came from. Hopefully more later.
Additional: Jumping back to the HRE, this map of the HRE in 1648, only 50 years ahead of my time of interest, (
https://upload.wikimedia.org/wikipedia/commons/8/83/HRR_1648.png) depicts the Obergimpern area as an unrepresentable small territory.