Is there any real explanation for this besides "Tiber Septim did it" which really doesn't work with the time line since that would have occurred before the pocket guide to the empire as created.
There was no explanation provided ingame. Truth be told the only reason we know that Tiber Septim used CHIM to change the climate of Cyrodiil is because Michael Kirkbride, an ex-dev who pops onto this forum occasionally, said so. His say so, however, is "questionable" because it was never stated ingame, and thus is not "official" canon.
Also to build on my post in the other thread on Cyrodiils transformation, the lack of an explanation for the change probably annoyed me more then the change itself.
Third, why do we have no real Imperial forts just ruins (And where are the living quarters, armories, etc.)?
This actually has a logical explanation, though it wasn't stated anywhere ingame as far as I'm aware.
Basically the reason for the lack of a significant Legion presence in the Imperial Province is probably because the General in command of the Legion troops in the Imperial Province could theoretically apply a lot of pressure on both the Emperor and the Elder Council through the threat of force or, should he grow power hungry enough, he could stage a coup d'etat and install himself as Emperor (side note, both threse reasons are why legions in ancient Rome, upon which the Empire is ostensibly based, were
never allowed past the Rubicon river). As it is there are just enough Imperial Legionaries based out of the Imperial Prison complex to keep some basic order along the roads.
Also the reason that the forts are run down is probably because they were either from the Second Era or the War of the Red Diamonds, both of which were periods when Cyrodiil was fragmented and prone to conflict, which would explain the need for fortified positions. Its likely that, once order in the province proper was unified the forts were emptied and fell into disrepair because the Legions were sent back to the provinces to maintain an Imperial presence.
as well as the sudden change in armor for the guards
Out of game, they wanted to make new eye candy. Same reason the rest of the armors were redesigned. Ingame, no reason is mentioned but this really doesn't rank anywhere near the top on my pet peeves with Oblivions lore.
Why do you care? It's a game and the world seems fine.
People care because the richness of TES's unique lore makes it both compelling, and unique from pretty much every other RP out there. When illogical changes are made without proper explanation it detracts from the lore, and lowers their gaming experience.