The more I think about grammar the more I'm sure, that the big shifts and changes coincide with big historical events. Same as with sound shifts, grammar shifts occur when language becomes a second language.
Compare it to evolutionary forces.
It's an interesting theory but some your examples can be easily dismissed.
Few examples:
English - a product of mixing few languages, at least two second languages. Latin was introduced over local population for 500 years, then germanic languages. As second languages they've never been learned in correct, original form. Keep in mind that Bretonic might have been already a second language for locals, after celtic invasions. At the end we have English, a language of simplest grammar, at least in Europe.
Old English replaced completely and utterly all previous languages in England, be it Latin or Celtic. Old English almost didn't have any loan word from Latin, except words like belt or tower. That's why historians long believed that the Anglo-Saxons had exterminated or force-exiled the ancient Britons to Wales and Cornwall, leaving a pure Germanic society (until genetics proved that it wasn't the case, so the native Britons all learnt Anglo-Saxon). Old English evolved into Middle English in pretty much the same way as Old Dutch evolved into Middle Dutch. The two languages were still quite mutually intelligible in the 11th century, despite 500 years of separate evolution. Both grammars started to show signs of simplifications without any outside influence.
Modern English resulted from the long brewing of Middle English with Norman French between the 11th century and the 16-17th century. The two languages progressively merged, but that doesn't explain why English lost its conjugation and grammar since both Germanic and Romance languages had them and have kept them to this day. English grammar kept irregular tenses only for basic Germanic verbs, but dropped the SOV (subject-object-verb) structure to adopt the Romance SVO. English also adopted the Romance plural in -s in replacement of the regular Germanic plural in -en and the numerous irregular Germanic plurals (except for a few common words like child-children or foot-feet).
So the reason English grammar is simple is not because of its ancient Celtic or Roman population, which didn't have any influence on the modern language, nor due to the fact that people were immigrants since there was virtually no change in population after the Norman invasion, and the Normans were only a tiny minority, and an educated one at that, so not one that would simplify grammar. It's hard to really pinpoint why English simplified its grammar, just like it's hard to know why the English like to do so many things differently from other Europeans.
Macedonian - it is an interesting case. It has the simplest grammar of all slavic languages, when compared to other slavic languages still with 7 declensions and very complicated grammar. It might prove that Macedonians didn't speak slavic in the past, and slavic is their second language, the language of invaders.
I don't know enough about Macedonian, but this example might be correct.
Latin - one of original EI language with complicated grammar. Vulgar Latin, Italian and other romance languages arose from Latin being a second language for conquered tribes of different languages. Again the source language got simplified with sound shifts.
Actually the only people conquered by the Romans who adopted Latin were the Italic and Celtic speakers, who spoke a language very close to Latin, the Iberians (who were sandwiched between Celtiberians and Romans), and the Greeks from South Italy who were just too close to Rome not to adopt Latin (although many South Italian communities kept speaking Greek deep into the Middle Ages, and Greek dialects are still spoken today in some remote villages in Calabria). The reason is that Latin was simply too difficult to learn for people who had an non-IE mother-tongue (like in North Africa and the Levant) or spoke Greek (which is only partly IE). Aromuns in the Balkans probably descent from the Celtic communities that had settled in the Balkans as well as Romans (from Italy) who moved to the region. Note that the Basques resisted Latinisation despite being surrounded.
I don't see how the grammar of Vulgar Latin could be influenced by Celtic languages since they had grammars with declensions much closer to Classical Latin.