sync all contacts between devices
You can use the CardDAV protocol to sync your contacts. Here is an explanation how you can use it: Syncing contacts

There is also the possibility to use Google as an Exchange account. You should be able to find the instructions on the interwebs (I fear, if I post a link it will be in german language)

If you also enable the Google account also on your Addressbook application, merging them should be easy (depending on the application). In most cases, you just have to drag and drop the contacts to the corresponding Google folder.