Mountain Lion download problems with Lion 10.7.5 (no VirusBarrier issue)

Question

I have a mid-2009 Macbook Pro. From the factory it came with Snow Leopard but has since been upgraded to Lion OS X 10.7.5. I do not have VirusBarrier, I am writing from a recently clean-installed Lion.
I have now been troubleshooting my Mac for several days. When I wrote first, I had an issue with Safari having been damaged and so I wasn't able to reinstall Lion - but after a certain extremely helpful community member I finally succeeded in installing Lion.
But the issues with the App Store still persist. All my Mountain Lion downloads fail, getting the error message "can't verify, might be corrupted" what many here have also been getting. However, most solutions were related to VirusBarrier (which I don't have), or /var/folders errors (Console does not show any var-related errors).
So if it's not a var-related issue, or related to firewalls or virus softwares (I have disabled the Firewall and I have no virus software installed), what could it be?
When the download and install of ML failed last time, Console showed the following messages:
11/30/12 11:54:44.617 AM com.apple.SecurityServer: Failed to authorize right 'system.install.app-store-software' by client '/System/Library/PrivateFrameworks/PackageKit.framework/Versions/A/Resources/in stalld' [235] for authorization created by '/System/Library/CoreServices/Software Update.app' [567]
11/30/12 11:54:44.617 AM com.apple.SecurityServer: Failed to authorize right 'system.install.software' by client '/System/Library/PrivateFrameworks/PackageKit.framework/Versions/A/Resources/in stalld' [235] for authorization created by '/System/Library/CoreServices/Software Update.app' [567]
11/30/12 12:25:49.252 PM authexec: executing /Applications/Install OS X Mountain Lion.app/Contents/Resources/InstallAssistantTool
11/30/12 12:26:17.178 PM Install OS X Mountain Lion: Stopped operation queue with Error Domain=IAErrorDomain Code=101 "This copy of the Install OS X Mountain Lion application can't be verified. It may have been corrupted or tampered with during downloading." UserInfo=0x7f93aad17ee0 {IAErrorAlternateAction=App Store, NSLocalizedRecoveryOptions=(
    Cancel,
    "App Store\U2026"
), NSLocalizedRecoverySuggestion=Delete this copy of the application, and go to the Purchases page of the Mac App Store to download a new copy., NSRecoveryAttempter=<IARecoveryAttempter: 0x7f93aab1e1d0>, NSLocalizedDescription=This copy of the Install OS X Mountain Lion application can't be verified. It may have been corrupted or tampered with during downloading.}
...and a bit later, upon re-starting the download:
11/30/12 12:29:40.515 PM com.apple.SecurityServer: Failed to authorize right 'system.install.apple-software' by client '/System/Library/PrivateFrameworks/PackageKit.framework/Versions/A/Resources/in stalld' [817] for authorization created by '/Applications/App Store.app' [781]
11/30/12 12:29:40.547 PM com.apple.SecurityServer: Failed to authorize right 'system.install.software' by client '/System/Library/PrivateFrameworks/PackageKit.framework/Versions/A/Resources/in stalld' [817] for authorization created by '/Applications/App Store.app' [781]
Problem elimination:
* I don't think this is a router-related issue: I have downloaded and installed other softwares outside App Store successfully
* As said, I don't think this has anything to do with Firewall or Virus settings
* Nor has this anything to do with /var/folders as far as I can tell
What I've tried so far:
* Repaired disk permissions several times
* Rebooted
* Restarted App Store
* Following one advise found on the net, I deleted preflight.pfpkg just in case
* All the time I have made sure my OS has the most recent Software Updates installed (most of them downloaded successfully, while with Safari and OS 10.7.5. I had to try several times before they were downloaded succesfully)
I just reinstalled my OS X Lion, only to use it to jump on to Mountain Lion - so I am pending my migration until ML is installed. I would like to fix this soon, since I'd like to get the migration done and my real work started. I would really appreciate your help.
I'm not very adept at using Console or Terminal to troubleshoot, so if someone could guide me through the problem identification, that would be great. Unless someone already knows exactly what's going wrong and can show me a working solution - that'd save my day!
Thanks!

Answer 1

In cases like this, if you have an Apple store near by, they will most likely install the OS free of charge for you, if you can't bring your Apple ID along to prove you've paid for it.
Would that work out for you?