On one hand, this is probably fake, because these kinds of leaks usually are.
That said, as far as these go, this one is not THAT farfetched. Things mentioned here are either in line with what we already know (amiibos unlocking costumes, new options for multiplayer, etc) or could feasibly be newly revealed this close to release (2D shooter minigame, campaign details, etc). Even the free download code sounds like what they did with MK8 last May.
If this is all true, it is generally good news. For one, since the campaign was not shown at E3 2014, I was worried that single player would be insubstantial. 28 levels at half an hour each sounds like a healthy amount, especially if unique mechanics are used well. I'm also reminded of Kid Icarus: Uprising, which had a similar number of levels clocking in at around half an hour each. If the gear in this game works similarly to that one, it would be a good way to pace levels and create replay value with different weapons. The difficulty settings will probably also be fun to play with.
The stages based on Super Mario Sunshine could sound a bit random, but I can easily see the style of movement and aesthetics of that game transferring to Splatoon. Maybe Sunshine HD was in the works back in the day before Wind Waker got the HD makeover instead? Probably not. Regardless, it also doubles as a good way to get attention from Sunshine fans.
The new online/mode-related stuff is basic but good. Team Death match is a standard mode, Battle Dojo ought to be online, Tournament mode would normally be a bit unlikely from Nintendo, but this does seem to be a leap forward for online play over there. The demo having an online stage would work well with that.
One odd thing about this is the 5 possible game downloads. The same thing happened last year, but NSMBU and Wii Party U instead of Tropical Freeze, 3D World, and Hyrule Warriors. The thing is, as downloads, those are odd choices considering their file sizes. Pikmin 3 and Wind Waker HD were 3.9 GB and 1.8 GB. Not that bigNSMBU and 3D World are both about 1.7 GB, and NSMBU is often bundled with the console anyway, so that makes sense. Hyrule Warriors and Tropical Freeze though are 8 and 11.4 GB, a large portion of the console's 32 GB of storage. Considering how no new version of the Wii U has been released with more storage, it seems a bit odd.
Love and tolerate.
