There is no official Google Hangouts app and may never be due to Google's nearly non-existant support for Windows Phone.
There is an API for developer to use so it's possible, but as you've noticed, no real fully featured 3rd party apps for Hangouts exist (yet?).
There is also no way to use a web version of Hangouts in Internet Explorer on Windows Phone.
The facebook chat option was removed. You can use the Facebook app to chat with your Facebook friends.
EDIT: The chat option has been removed from the Facebook app. You need to install the Facebook Messenger app.
There is no official Google Hangouts app for Windows Phone.
You can use IM+ or Agent + ICQ apps. They are not official or fully functioned but still better than nothing.
See http://www.guidingtech.com/29720/gmail-maps-gtalk-windows-phone/ for review.
See reddit thread for discussion.
The backup of whatsapp in Windows devices stay local , i.e; they get saved in sd card or in internal memory unlike Apple and Android devices, where backups go to cloud(idrive n Google drive respectively).
Before uninstalling the whatsapp, you must check for backup(whatsapp automatically backs up the messages from time to time), then next time while ...