No, only Etisalat users should subscribe to any of the Internet Calling Plans, whether Mobile or eLife. The users outside of the UAE, whom you want to call or text, only need to download the same supported app.
Yes, you can subscribe to both types of the Internet Calling Plans at the same time and you will be charged for each plan separately.
If the contact number is registered to more than one eLife account, the request will be rejected. You can use any other activation channels.
You will lose ICP add-on and you should subscribe once more after migration takes place.