I think it was already running pretty smoothly at E3 last year, if it was rushed it'd be buggy, but that doesn't appear to be the case, or I haven't read anyone say they found it have any of those kinds of issues. It just seems the things you're mentioning (OP) are Nintendo's choices for the game.
For the kind of game Splatoon is I don't think it's being rushed, from Wikipedia it says this:
http://en.wikipedia.org/wiki/Splatoon
"In mid-2013, one member of the Animal Crossing development team, Shintaro Sato, created a four-versus-four paint-based territory control game set in a featureless arena.[8] After management approved making the prototype into a full game, the team started expanding its concepts."
So it's around 2 years in full production by the time it comes out in May this year.
If I remember correctly a game like Toad Treasure Tracker only took a year to develop. While Splatoon is a new IP, it's assets aren't particularly complex in design, although it does have a nice art style.
As for the Voice Chat issue, I think that may be because Nintendo are concerned about kids and adults interacting, basically them trying to be the parent and prevent any harmful abuse by adults to kids.
The Bots thing could be a technical issue, although I don't see why Wii U can't handle AI for Bots in a multiplayer, PS3 and 360 did in some games.
The 4V4 thing could just be what they feel is the best dynamic, without things getting too hectic or messing with the kind of pace they're trying to set in their game.
The lack of info about certain aspects of the game is like any other developer trying to hold back some details so they can keep people from knowing the full story until they feel it's time to bring that out for their marketing campaign for the game.
Nintendo can always add things in DLC or make additions in the future, if the masses aren't happy with how the game pans out. I don't think Nintendo have done many online centered multiplayer games, but really I think much of these things the OP sees as issues are just subjective choices Nintendo have made for their own reasons and they're trying to make this their own style of shooter.
Personally as a more single player focused gamer I would like to find out something about that, info is pretty thin on the ground unless I've missed something about it.
TBH considering the game has been said to have Single and Multi Player, One on One local multiplayer, unless the MP only features a few maps and the Single Player is really short I wouldn't say it's been rushed.
Not much is known about the level of content yet as far as I'm aware.
I guess we'll find out pretty soon.
