Since you haven't finished ME3, I won't go in too much about the ending except to say they **** you the player over it. There is some seriously massive backlash over it right now.
Constructing a universe like that with a trilogy in mind isn't that far out and in fact isn't that unusual. I point you towards
Babylon 5 written by J. Michael Straczynski. It was written with 5 seasons in mind which ended up extended to a 5th semi-unplanned season due to the network they were on collapsing around them during the 4th. Story telling of that scale has existed for a long time and it's only recently has
games caught up to some extent. For games it is far easier to do since you can include all story line possibilities assuming you have and open game oppose to having to decide what to cut out. Also once it is written, the story effectively plugs into the game. Sure the developers still have to make the assets and so forth, but they don't have a weekly production deadline to come out with an episode every single week.
For a TV production you have write in character trap doors for when you need a person to disappear from the show without disrupting the narrative. Actors working on multi-year projects can become sick, die or have unresolvable personal conflicts with the staff or commitments that require work arounds. Games don't have that problem. While there is some uncertainty with whether ME would get a full trilogy, that wasn't the writers problem. If it never made it to a full trilogy the ending would have been whatever the last game's ending would have been and if your lucky you get a DLC ending. The challenges faced by the ME writers pale in comparison and really do a disservice to far superior writers and the difficulties they faced by elevating the ME writers like that. Videogame story telling is improving, but still very much in it's infancy.
Having a minimum of 2 tablets for the WiiU is a no brainer. This is the same situation with Motion Plus which should have been included with the Wii out of the gate, but Nintendo cheaped out on. While wireless bandwidth would be a problem, they already have a solution. The second tablet can be hooked up via USB. It is a simple and elegant solution. In a future revision, both can be wireless, but right now, one wired and one wireless is a good compromise and importantly, feasible and cheap to do. I would be extremely surprise if they don't have a second tablet as it is a very boneheaded move to have only one.
Input lag should be much of an issue since the controller still functions as a controller and it simply picks up on the broadcast. Onlive as much as I don't think it is the future of gaming has already shown that encoding of the video is very fast and can fit into a very limited pipe, one more limited and error prone than a couple meters of air. It becomes a question of decode speed, bandwidth and battery life. Use more bandwidth, you can decode faster and use less battery power. It's only a matter of trail and error to see what the acceptable envelope is for all three factors. The console won't be the problem as that will no doubt have a dedicated single function chip to handle the encode at a very high speed not too dissimilar to the ANA chip for scaling found in the 360.
As for games like Mario Kart and F-zero, I don't see a reason why they could exist at the same time. They are both racing games, but they are very different racing games. It's like saying because you have Mario, you can't have a Donkey Kong platformer. It's absurd. It's a question on return on investment. Between MK and F-Zero, F-zero has a lower return, but it a positive return. Nintendo only has so many resources to expend for one reason or another, willingly or otherwise, so will tend towards the low hanging fruit. As a Franchise lies fallow, over time the return on investment increases as that segment remains unserved by new games, so like farming becomes increasingly fertile. Eventually you rotate it back in and get a bumper crop.
This is the opposite of other publishers *Cough*Activision*Cough*, which practices slash and burn which eventually reders the market segment worthless. Or Rovio with it's mono crop which is extremely exposed with one single point of failure and the fact they are not a games company, they are in the business of marketing trading on momentum alone.
This is why even Zelda with it's tendency having 2 games per generation tended to have one game that is considered Gaiden or gameplay wise somewhat out of line. It was the same "Crop" say corn, but it wasn't planted in the same field and was a different variety.