Well I just recently bought a brand new copy of Oblivion for the Xbox 360, the standard edition since I'd had installed Shivering Isles 3 years ago....I put the disc in, and when it went to the main menu, I loaded my last save file which had 184 hours or so on it, Save 1157...it said something like 'some content is missing, some will not load when loading save file' or something along those lines, 'continue loading save without content anyway?' sorry If i'm not remembering the exact details...

so I selected load, and just to test things out I looked for the Knights of the Nine building and the Shivering Isles teleport and could not find it anywhere. I also could not find some of the gear I had gotten from Shivering Isles quests in my inventory nor chests...I know it's been well over 3 years since I last played it, but I specifically remember saving my last file in my own home in Skingrad...I loaded the save file, I'm in my Skingrad home, but none of my Shivering isles or Knights of the Nine armor is to be found...what the heck?

How could anything POSSIBLY be missing if Shivering Isles is still INSTALLED on my HDD? Do I need to install Shivering Isles again because I may somehow have an older version of Oblivion? If I do all that, is my save files corrupted since I loaded without any SI or KoTN content? Can I get my stuff back? I actually came back to finish about 10 or so more quests (100% completion on the UESP wiki actually says something like 199 quests in total in the game, and I've completed 189 quests) I like to complete any Elder Scrolls or Fallout game before moving on to the next one, or else it's impossible to play without thinking of the previous title.

The whole time I cannot immerse myself in Skyrim because I feel like getting the last couple achievements by completing the Dark Brotherhood questline, and finding out what happens in that questline, it's the only major quest line I had not completed and so on and so forth.

You may call me crazy for wanting to finish Oblivion before fully getting into Skyrim but just so you know I've put 115 hours in Skyrim already...haven't done any major quest lines in Skyrim however.

TL;DR; can I get my stuff back by installing the expansions again, even though I'm pretty sure I already have them installed? Why would it be missing again?

The expansions need to be activated in order for their content to appear. Just because they are installed, doesn't mean they're active. Activate SI and KotN, then reload your last save ( 1157 according to your post), and the missing content should be there.

how do I activate it?

hellooo?

I don't know how it is for Xbox, but I know that on PC, you have to install the expansion content after the main game, because it makes patches to the original game.