The Macedonian people are largely Slavic and Albanian in origin and speak a South Slavic Language with deep ties to Bulgarian, despite having few ties to the historic Kingdom of Macedon or the ethnic Macedonians (who identified as Greeks or Hellenes often) of Greece and occupying a territory north of the Macedonia they identify deeply with that ancient kingdom and use the name. What caused the Macedonian people and government to adopt the Hellenic Macedonian identity as their own and try to link themselves to them historically? I understand this is a hot issue in some circles and apologize for any mistakes I've made or offense I've caused, but I am genuinely confused by this.
To clarify the linguistic issue, Macedonian has "deep ties to Bulgarian" in much the same way that Danish has "deep ties" to Swedish: in both cases we have closely related languages existing in close proximity with a great deal of internal dialectal variation, and neither in each pair of languages 'derives' from the other or is somehow prior.
I don't think there's a whole lot to say specifically about the adoption of the name Macedonia, except that it was a geographical term in use in the 18th and 19th centuries.
To give a condensed version of Victor Friedman's 1975 article on early Macedonian nationalism, South Slavic nationalism started to become a thing in the late eighteenth century, and early on South Slavs in what's now Macedonia and Bulgaria were fairly united in forging a literary language that wasn't Hellenized, but split on what region's dialect to use as a base. Macedonian nationalists came in both unitarian and separatist flavors, as Friedman puts it. Unitarians wanted a single Macedo-Bulgarian standard with some significant Macedonian influence, while separatists felt that their language was too distinct from the nascent Bulgarian literary language for for a unified standard to be usable. On the Bulgarian side, compromise looked like Macedonians adopting the standard based on dialects spoken in Eastern Bulgaria, and Bulgarian philologists and linguists maintained the position that Macedonian was a totally fabricated 19th- or 20th-century corruption of Bulgarian until quite recently. It took some time before Macedonian nationalists really began to be explicit about the project of a national Macedonian language, since doing so would alienate presses and governmental authorities, first in the Ottoman Empire and later in Bulgaria and Greece, though Serbia was somewhat more permissive about the issue.
As far as structural differences between Bulgarian and Macedonian, Macedonia has the misfortune in an era that defines nations as languages to sit astride a bundle of isoglosses that fan out from the north-eastern corner of the country. Isoglosses are geographic boundaries for a given linguistic feature. Often we see several of them line up together, and they produce a situation where people from areas on either side can't understand each other, but just as often we don't, and there is a gentle shading of one variety into another over tens or hundreds of miles. In a 1985 article, Friedman describes the situation quite well:
There is a bundle of significant isoglosses running roughly along the Serbian-Bulgarian political border, but at the Macedonian-Bulgarian political border these isoglosses fan out across Macedonia, so that while the transition from Serbo-Croatian to Bulgarian is relatively rapid, that from Serbo-Croatian to Macedonian to Bulgarian is very gradual.
While the linguistic situation in Macedonia is rather complicated, with basically any feature you want to pick showing different dialectal distributions, there are still Macedonian features we can find even in our earliest Slavic manuscripts. As noted in the introduction to Horace Lunt's 1952 grammar of Macedonian (linked below), the Codex Zographensis shows characteristically Macedonian reflexes for certain Old Church Slavonic vowels.
So although it's not at all the case that Macedonian was invented out of whole cloth by Serbians after WWII, it wasn't really standardized until after WWII. As Friedman discusses, the West Central region was agreed upon as the base for the standard language. The first commission for a standard orthography produced a proposal that looked very Russian (and, unintentionally but more importantly at the time, I think, Bulgarian) in character. The second commission produced a second proposal that was more Serbian in nature, though the letters ѓ ѕ ќ /ɟ dz c/ were particular to Macedonian. About 7 years later, Horace Lunt produced an English-language grammar of Macedonian (large pdf warning) in consultation with Macedonian scholars involved in codifying the literary language.
All of this is to say that while Macedonian and Bulgarian are closely related, and Standard Macedonian and a separate Macedonian identity are somewhat younger projects than their Bulgarian counterparts, it's really not the case that Macedonian or Macedonian identity derive from Bulgarian or Bulgarian identity.
[…] and occupying a territory north of the Macedonia […]
I have a related question. Has the area connected to the name Macedonia changed over time? Did it ever (before the modern country) include most or all of what is now the F. Y. R Macedonia?