Leaving Japan without return residence card

Hi all,
I have "Activities designated" visa valid up to December.
I am planning to go back to my country in July and I don't want hand out my residence card because there is a chance that I will be back before December.
For that I have 2 concerns
First: in case I decide not to come back to Japan. Can I return my residence card from abroad? I read that we can send the card by post but I am not sure. how about the health insurance cancellation? I am planning to pay everything before leaving.
Second: I will move out from my place in July. Can I put a friend's address when moving out and not register at the city hall where he lives. This will not cause any troubles for him?
Your time and answers are much appreciated.

Probably best if you just contact immigration and ask them.

Second: I will move out from my place in July. Can I put a friend's address when moving out and not register at the city hall where he lives. This will not cause any troubles for him?
You need to have a registered address, so if you move out and don't change within 14 days that is going to leave you registered at a place you don't live . And putting your friends address - that would be the one you notify to city hall (and goes on your residence card). How else will the city know where to send your tax bill to (because you are still liable for that).