Newby to Linux and Elementary. Have tried Ubuntu. I find it very frustrating trying to install apps. Two in particular; Opera and Libre. They seem to do the install thing, but never appear in the installed apps. Great interface, but I suspect it's time to punt Elementary and move on.
I can confirm that both Opera and Libre install just fine. I can also confirm that some apps do not show their proper icon when first installed and the install was not made via the appcenter, however after you do a reboot you will see the proper icon in the application menu for your apps. To install Opera I downloaded the .deb file from their website and used eddy to install it. Checked the application menu and the app was there without an icon but worked just fine. Libre is an easier install as you just have to install from the appcenter and it will automatically appear in the application menu.
Hope that helps. Best of luck.