Yes, local accounts are those that exist - are/have logged in - on the same console, whether created there or elsewhere.
The Playstation Plus subscription is tied to the SEN account, but that account can specify a single PS4 console as its primary PS4, which enables other (non-PS+) accounts to access everything on the console.
So any account using that console (the one specified as the primary by at least 1 PS+ subsriber) can enjoy the benefits of PS+, when using that console. That includes multiplayer.
I assume though that multiplayer is not available to local accounts if you are online and playing multiplayer on someone else's console. That part isn't exactly clear. But your (non-PS+)family members / housemates could definitely play multiplayer on your console with only your PS+ subscription when you're not using it.
This FAQ on playstation.com is fairly detailed:
PlayStation 4
You can activate one PS4™ on your Sony Entertainment Network account
(this is known as your Primary PS4) and temporarily activate a second
PS4 on your account.
PlayStation Plus benefits available to other non-subscribing users on your Primary PS4:
- Online multiplayer (subject to any parental control settings on sub-accounts)
- Play downloaded games purchased with PlayStation Plus discount
- Play downloaded IGC games
PlayStation Plus benefits not available to other users on your Primary PS4:
- Purchasing PlayStation Plus discounted products from PlayStation Store
- Online game save storage
- Auto patch download
- PlayStation Plus exclusive early access to game trials
PlayStation Plus benefits available to you on a non-subscribing friend’s PS4:
- All of your PlayStation Plus membership benefits are available to you when you use a friend’s PS4.
PlayStation Plus benefits available to your non-subscribing friend when you use their PS4:
- Play downloaded games purchased with PlayStation Plus discount
- Play downloaded IGC games
PlayStation Plus benefits not available to your non-subscribing friend when you use their PS4:
- Online multiplayer (subject to any parental control settings on sub accounts)
- Purchasing PlayStation Plus discounted products from PlayStation Store
- Online game save storage
- Auto patch download
- PlayStation Plus exclusive early access to game trials
When you sign into your Sony Entertainment Network on a friend’s PS4
the system is temporarily activated on your account. Please ensure
that you correctly sign-out of “PSN” on your friend’s system to keep
your account secure.
If your “PSN” account is inactive on your friend’s PS4, the system
will be deactivated automatically from your account and your friend
will lose access to any of your downloaded content. This content can
be deleted from their system hard drive by going to Image (Settings) >
System Storage Management > Applications. Next, highlight the game to
delete and press Options > Image (X button).
Source: http://faq.en.playstation.com/app/answers/detail/a_id/5030
For primary accounts on a console there can only be one per account. When you make a new PSN account on his console this will make that PS4 be your primary console (I believe it does this for brand new accounts). You will not be able to do cloud saves without paying for playstation plus, but your trophies will be saved to the cloud by default since it is related to your PSN account. To answer your questions directly:
This will not damage your friends PSN account.
This will not do anything wrong to his account or PS4, though when you get your own PS4 you will need to deactivate your account as primary on your friend's PS4 (if by chance it is primary) or you will not have access to any purchases you may have made or will make.
Best Answer
If you activate a PS4 as "primary" for a PSN account then you can play any game that account owns regardless of what account is currently signed in to the PS4.
For example, if you purchase "The Walking Dead" with your Mexican account, and that account has activated the PS4 as "primary", then you can sign in with your Spanish account and play "The Walking Dead", earning trophies, using DLC, etc. with no limits.
Note that if you were to sign into some other PS4 with your Mexican account, and not make that PS4 primary (for instance, at a friend's house), you could download the games, but no other account could play them.
You can only have a single primary account.