At the last few decades/centuries of the HRE's existence, did it exist in any way other than in name, as in, did the Emperor have any actual authority and were the HRE's states subject to a central authority, and if not, when did it cease to truly exist, and would that mean that it's 1800+ states were all independent nations at some point?
I'll quote an answer of mine to a very related question from some time ago:
In the 16th century the Holy Roman Empire transformed in such a way to make it very difficult for us nowadays to really understand it. The Habsburg dominions were expanded with Hungary, which was not part of the HRE. And with the peace of Augsburg (1555) the empire was recognized to have both Protestant and Catholic constituencies, as decided by the local prince or city council in the case of free cities.
From this point it makes no sense to see the HRE as a state. The Habsburg dynasty produces emperors of the HRE, but in practice they rule Austria-Hungary, not the HRE. The thirty years war only entrenches the divisions further. Habsburg control over the rest of the HRE is nominal, not practical. They are definitely involved in its politics, in as much as it has politics.
But it's less like the European Union and more like the United Nations at this point. An organization with high aspirations and very little actual activity. In the period 1648-1804 the HRE means essentially nothing. It is in this time that Voltaire quips that the Holy Roman Empire is not holy, not Roman and not an empire.
Does this mean that all of the entities in the HRE should be seen as independent states? Not exactly. Quite a few areas were rules by abbots and bishops, who were subject to the pope as well as the emperor. And imperial cities and small principalities or fiefdoms with imperial immediacy would be very aware that their special status was in some way still connected to the old imperial system, as much of a sham as it was when it came to the relationships of the largest principalities.