Heya, you might find a more receptive audience to discussing the creation of an alt historical universe at /r/worldbuilding. One of the reasons you're finding such resistance here is your insertion of a New And Special (TM) land into Europe. While it's really important to stress the diversity of individual contexts--and I mean down to individual cities, not just kingdoms and centuries--at the same time, even medieval Europe was much more connected philosophically/worldview-wise than this idea of a "Roman holdout" could support. Sandwiching a bunch of knowledge of classical antiquity between the Alps and the Rhine just couldn't happen. The trend we see, in actual history, is that the slow disintegration of Roman centralization shifts the norms and needs of the governing classes away from literacy and higher learning. Although there are earlier classical survivals and revivals, it's really in the 11th-12th centuries that we see ancient philosophical, proto-scientific, legal texts either having an impact or present at all in western Europe. This comes especially through the translation efforts in Iberia and Sicily. Throwing a pocket of classical antiquity into the mix in, say, 800 would drastically alter the intellectual situation in medieval Europe. The ramifications of that--the reliance on texts, on written case law--would fundamentally alter European history going forward. By the time you reach your actual period of 1600, Europe would probably look rather different than it did in actual history, particularly the relationship between religion and state or proto-state.

/r/worldbuilding can help you work out the implications of adding a New And Special country to Europe.

Here is an excerpt of a little bit of history I already have written

After the victory of the catster army over the previous republican regime, the generals were left to discuss the formation of the new government. An assembly of major revolutionary political figures was called, and Pierre Meister was given a golden invitation. He was a figure whom the old regime despised and was in prison for writing radical pamplets when the revolution started, so it was only natural to invite him. During the council, he gave his opinion on several of the issues being discussed, including the death penalty, tariffs, indentured servanthood, and democracy. The ideas he voiced there are still respected today and codified into Nioyan law.

A consensus had built up that there must be one single ruler, but no major preference existed for what form the new monarchy should take. A discussion was held as to who the new king should be. Many were named as possible candidates, and one suggested was Meister. At first he declined, as he did enjoy being well known. But the assembly insisted. Even one Marius Elephantus said "only one who rejects the offer should have the position!" In the end, the decision was up to the generals of the catster army who had control over the government and armed forces. Finally, Pierre was chosen. After the decision was made, "Huzza, Huzza" was shouted by the whole room. Soon his name was printed in every newspaper and the whole nation rejoiced.

Meister was given the privilege of drawing the structure of the new state. He called together a company of experts and philosophers that he believed could give him valuable advise. Interestingly, a first set of advisors was called and Meister expressed dissatisfaction with the meeting. He found it inadequate. He said the advice given was wise, but the teachings he received did not resonate with the soul of the people. Everything discussed was too lofty and abstract. He called together a second council of advisors, this time being filled with poets and romanticists. A second draft of the new constitution was made, and Meister felt content with the new suggestions/

At the very end, a council of bishops were called. He announced that a new king was ruling the land, and that the religious authorities should give him the honors due to him. They modified the ancient ritual for the annointing of an emperor, and Meister was given the blessing of the patriarch as "The Prince of the northern Sealand". He then made a coronation oath written by himself and one of his advisors. This date marks the birth of the new nation in Nioyan history books and is celebrated as a holiday in the country.

Advice, corrections on anachronisms, suggestions for better terminology, suggestions for better historiography, anything that ought to be corrected or made better are welcome.

You have to be a little bit more specific than "Europe from 1600 to 1914". That's my area of study, to be loose, and just on a focus of France and Germany I have nearly 100 books on my wall next to me + articles on my computer + eBooks on every topic from genocide, to economic history, to military history (from operational histories to more specific aspects of specific armies in specific periods), to social history, religious history, technological history, and so forth.

So, needless to say, please specify down a bit here. Though the idea of an absolutist regime existing past the Enlightenment and especially the French Revolution will be nearly impossible to justify without flat out ignoring those events existence.