They'll lose access to the PS+ perks, yeah. If you leave the account there as a secondary one, they can at least still play any of your digital PS4 games by having your account stay logged on,
Yeah, you can deactivate a PSN account from anything remotely via the the accout settings page in the webstore.
Err, kind of. You have to either manually deactivate the primary console on the console itself, or you can use the webstore and force a deactivation on everything that the account is activated on, but you can only do that once every six months.