I am from Europe and have a US PSN account. If I buy Playstation Plus through the PS Store, will I be able to play online multiplayer games through that account as I'm not from US?
Yes, it will work. You can buy PSN cards online, such as on Amazon, but make sure you purchase a US PSN card. Once you've purchased it, a code will automatically be sent to your email. If you have other accounts and want to buy PS+, all other accounts will also have PS+. The same goes for DLCs and downloaded games. However, this can happen only if the US account is the primary account.
In addition, make sure all accounts are set up before buying PS+. I made this stupid mistake of making my US account, purchasing PS+ and then creating a new user, so that new user didn't get PS+ because I created it after buying PS+ on the primary account.
I created this guide for creating an US PSN account a while ago. Although the screenshots are now old, the process is still valid. Essentially, you need a Virtual Credit Card to use on the US Store, and just sign-up using a fake US address.