The Apple Support Communities are an international user to user technical support forum. As a man from Mexico my first language is Spanish. I do not speak English, however I do write in English with the aid of the Mac OS X spelling and grammar checks. I also live in a culture perhaps very very different from your own. When offering advice in the ASC, my comments are not meant to be anything more than helpful and certainly not to be taken as insults.
After installing Mt Lion, the installer self-deletes when the installation finishes. The MAS expects to see it in the Apps folder. Because it isn't there, the MAS is ready to allow you to download it again.
When I go to Mountain Lion from welcome page, Mountain Lion says to downlaod.
It doesn't necessarily mean you must re download Mountain Lion.
You can use the Download button to Create an OS X Mountain Lion Installation Boot DVD or USB Drive with LionDiskMaker
As for updates, sometimes signing out of the App Store then signing back in can help.
This has been an App Store anomaly for awhile but there is a workaround.
You can delete an app using this free utilty >Download AppCleaner for Mac - Uninstall your apps easily. MacUpdate.com
That deletes the app and all the associated files.
Then re download the app from the App Store. That should install the lastest version avaiable without seeing Updates available.
How to re download apps from the Mac App Store:
Open the App Store. From the menu bar click Store > Sign In
Click Purchases from the top of the App Store window.
Select which apps you want to re download. Then right or control click where you see Installed then click Install.
Make sure and use the same Apple ID used for the original purchase.
You can submit feedback to Apple if you'd like. You're not the only App Store user that has reported this problem.