No you cannot use them. Forever stamps can only be used for domestic (inside the US) mail. They must have a value on them for international mail.
no
No. It's enough to write "Par Avion - By Airmail" clearly in the top left hand corner of your envelope.
Yes, they are valid 'Forever' for one ounce of first class postage.
You can use forever stamps on the letter. It is worth 44 cents toward the cost. You must put additional postage on the envelope.