Can't update Xcode – "You have updates available for other accounts" and other errors.

Question

I have 2 Macs, a 2008 Mac Pro running 10.8.5 and a 2009 MacBook Pro running 10.9.0, soon to be downgraded to 10.8.5. I can't install the Xcode update on either Mac, and I can't update MacTracker on my Mac Pro, which suggests that no updates work at all.
When I try to update, it says "You have updates available for other accounts" and "Sign in to ________ to update applications for that account." I'm already signed into that account. I tried logging out, closing the App Store, and logging back in. Then I tried logging out of both the App Store and iTunes as per a fix I found online, and it gave me "Unknown error" once I tried to update.
Is there a fix for this or at least a regular .pkg I can just install? Honestly, it's easier to update stuff in Debian Linux than on OS X 10.8 and up now.

Answer 1

Please try each of the following steps you haven't already taken, until the issue is resolved. After each step, relaunch the App Store and try again.
1. Follow the instructions in this Apple Support article.
2. If any App Store purchases were made under a different Apple ID, or if the bundled iLife applications were accepted under a different ID, sign in to the App Store with that ID to update.
3. Alternatively, delete the probem applications and re-download them with the ID you're using now. If you haven't already purchased them with that ID, you'll have to do that first, because purchases can't be transferred from one account to another.
4. Select the Purchased tab in the App Store window. If you see an item in the list marked "You have 3 apps to accept," click Accept.
5. Triple-click anywhere in the line below on this page to select it:
/Library/Caches/com.apple.appstore/adoption.plist
Right-click or control-click the highlighted line and select
Services ▹ Reveal
from the contextual menu. A Finder window may open with a file selected. If it does, delete the selected file. You may be prompted for your login password.
Then repeat step 4.
6. Select
Store ▹ View My Account
from the App Store menu bar, then select View hidden purchases. If you're trying to update any hidden purchases, unhide them.
7. Launch iTunes and select
Store ▹ Authorize This Computer
from the menu bar.
8. If you've moved any App Store apps out of the Applications folder, or into a subfolder, put them back. Never move or copy those applications. If you have backup copies of the apps on another volume, unmount it and try to update again.
9. Contact Account and Billing Support.