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So, these are some huge questions to begin with. But let me start with the following: Without being overly familiar with Mélenchon's exact position on these issues, there was and is a certain strand of a European and US American hard left out there who have turned their opposition to external influences in the Yugoslavia of the early 90s and subsequent actions by various Western European and the US government within the Yugoslavian conflicts into a veneration of Slobodon Milošević, Serbia, and even actors of the Republika Srpska despite their actions in the lead-up to the war, the dissolution of Yugoslavia, and of course in the campaigns of ethnic cleansing and genocide during the war. Reddit user /u/JFVarlet has written extensively about this in various threads in the badhistory sub such as here on Ed Herman; here; here; here; and here. Their posts cover quite a lot of ground and while Herman who is mentioned frequently was one of the most outrageous cases of this phenomenon, he was far from the only one – Austrian novelist Peter Handke comes to mind – and while indeed one of the most extreme cases, still indicative for a larger trend among certain circles within the international left: Mistakenly seeing Milošević and Serbia as the continuation of a socialist, multi-ethnic Yugoslavia despite the fact that it was him who literally presided over said Yugoslavia's destruction. Again, without knowing the exact history of Mélenchon's positions on these issues, I have a rather strong suspicion that while he might not have gone full Ed Herman so to speak, he too was affected by this twisted logic of opposition against Yugoslav disintegration and Western influence in the process turning into Milošević-apologia and a rose-tinted view of the role of Milošević's Serbia in the disintegration and subsequent war.

As for your questions, there is a lot of ground to cover but one can easily be answered very quickly: "was Slovenia also a Nazi ally back in WW2, similar to the Independent State of Croatia?". The resounding answer to that is no. Slovenia was annexed in part by Italy but for the most part by Germany and became part of the Reich. It was subject to a policy of forced Germanization (in accordance with the Habsburg inspired theory that Slovenians were in part Slavic speaking Germans rather than Slavs), which in practice meant the deportation of Slovenes and Serbs living in the territory to German-occupied Serbia, massive discrimination against Slovenes, including the prohibition to speak their language to children being taken from Slovenian families in order to be raised German somewhere else in the Reich. The relationship between occupied and annexed Slovenia was nothing like the satellite-state relationship with Ustasha Croatia and to call the Slovenes "Allies" of Nazi Germany is a massive misrepresentation of history. France in form of the Vichy government was more of an ally to Nazi Germany than Slovenia was during the war.

Moving on to the issue of Slovenia and Croatia in the late 80s and first two years of the 90s and their policies surrounding the disintegration of Yugoslavia. The German doyen of historical research on the history of Yugoslavia, Holm Sundhaussen, has identified the reasons for the disintegration of the Yugoslav socialist state in a conflation of crises that were all interlocked and depended on each other but can be described as

• the economic crisis – the economy had been in a bad state already when Tito died due to a variety of reforms instituted in the 70s not panning out;

• the intellectual crisis – in search for a new answer to what constituted Yugoslav legitimacy in the first place and in a situation where socialism faltered around Europe, intellectual and politicians increasingly turned to nationalism;

• the Kosovo crisis – Kosovo had experienced a massive modernization in Yugoslav times and with new wealth and education had come a generation in the 1980s that demanded Kosovo to become a full-fledged Republic within Yugoslavia triggering a question that had been to a very large degree avoided since 1945: What was the status of Kosovo within socialist Yugoslavia, which in turn did trigger nationalist sentiments especially in Serbia.

• the political crisis: Tied with the other crises and in part due to the conflict between Republics and between reformers and old hands, reforms became increasingly impossible within a Yugoslavia where a central federated government held extremely little power compared to the Republics that formed it. This was in large parts what enabled Milošević and some others to gain power and trigger what was known in Serbia as the "anti-bureacratic revolution" of Milošević, filling important government roles with Serbian nationalists, especially in Montenegro.

While the details of how many of these crises affected Serbia in the rise of Milošević, the increasingly sharp rhetoric towards Kosovo, the beginning of the genocide discourse (Albanians in Kosovo having more children than Serbs thus "outpopulating" them and thereby commiting genocide against the Serbs), etc. pp. are well known, how these crises affected the other republics goes to the very heart of your above asked question on Slovenia and Croatia.

As Sundhaussen points out in his book Jugoslawien und seine Nachfolgestaaten 1943-2011, a lot of works in the past have focused on the Serbian-Croatian antagonism as the driving force behind the disintegration of Yugoslavia but this is not warranted as in the Serbian-Croatian antagonism becoming dominant in the 90s while the driving antagonisms of the late 80s was the Serbian-Albanian (as is known) as well as the less known Serbian-Slovenian antagonism.

Slovenia and its population had initially been in massive support of the Yugoslav project. Considering their position both after the First and Second World War, Slovenian participation in the state of Yugoslavia had been seen as absolutely essential in order to protect country and population from interests of their neighbors, especially Italy and Austria. Especially in socialist Yugoslavia many Slovenian politicians had been very important for the concrete formation of the state itself and for Yugoslav socialism, among them Edvard Kardelj, Boris Krdič, Stane Dolanc etc. pp. But around 1987 this pro-Yugoslav sentiment seemed to disappear to replaced by a new Slovenian nationalism. The reasons for this are numerous but among them were that Slovenia and its population saw no more danger emanating from its immediate neighbors, that as the richest republic, they felt they had to finance poorer republics with their money, especially in a an inefficient system, and that Milošević's anti-bureacratic revolution created fear among the populous that a Serbian hegemony was in preparation within Yugoslavia.

So while with Milan Kučan taking power in Slovenia as both head of government and head of the communist party, antagonism between Serbia and Slovenia reached new heights and a much stronger discourse (in part inspired by the Slovenian expatriate community around the world) started taking place, Slovenia also was in a rather unique position that it was one of the most homogeneous republics of Yugoslavia and that a very small number of Slovenes lived in other republics. What that meant in practice was that when "Slovene" and "Yugoslavs" started becoming incompatible and when it suddenly became important again, who was a Slovene, a Serb, a Croat and so forth – I talk about the problems with this in this answer – unlike in Croatia, Serbia, etc. there wasn't suddenly a community of alleged "others" in Slovenia unlike in other republics. In this sense, the national aspirations of Slovenia gave less reason to be concerned than e.g. those of Serbia in the sense that within Slovenia there hardly was anyone to ethnically cleanse and thus such a program was simply not necessary to prepare.