I'm not sure why you would want to get a Japanese WiiU. Yes, the system is region locked, but there aren't many (any?) WiiU games that only got released in Japan, and tracking down Japanese WiiU games will be considerably more expensive than tracking down the domestic versions.
But anyway, here's the answers to this first part: A Japanese WiiU will require you to create a Japanese Nintendo ID account. It'll ask in Japanese, which prefecture you live in, what your address is, and your credit card information for eshop access. Note that American creditcards CANNOT be used. If you don't have a Japanese creditcard, you'll need to use Japanese Nintendo Points cards. Points cards like games, are region locked. You cannot go to a store in the US, buy Nintendo Points, and then use them on a Japanese WiiU.
A Japanese WiiU will not connect to NoA, ever. The reverse is also true; American WiiUs will never connect to NoJ.
Services like Netflex will be restricted to whatever the Japanese have access to. You'll not be able to gain access to the American Netflex.
The main drawback of buying a Japanese WiiU is not being able to access anything on the North American eshop including sales and promotions.
Nintendo of America will not service a Japanese WiiU. Period.
I can't speak to input lag because it's never been a problem for me, but just know that the WiiU contains a Wii inside of it so I wouldn't expect anything different from using the original Wii. But again keep in mind that the Wii is region locked as well. A Japanese WiiU will only be able to play Japanese Wii games.
The WiiU will work with the Wiimote, Nunchuck, Wii Classic/Pro Controller, WiiU Gamepad tablet and WiiU Pro Contorller. No GameCube controller accessories or any USB controllers or controller adapters. Nintendo did offically release a GameCube adapter for the WiiU, but it ONLY works for Smash Brothers 4. Also, the WiiU tablet and Pro controller don't generally work with original Wii software, and the original Wii Classic/Pro controller don't work with WiiU software.
The WiiU is designed for widescreen modern TVs (thank god) so anything less than HDMI is shortchaning yourself, especially using a 4:3 CRT relic. You'll run into all manner of overscan and wrong resolution issues using an old tv with S-Video. My advice is don't. But yes, the WiiU carries over the same connection port as the original Wii for SD video output, so you could track down an original Wii S-Video cable. But don't. The power brick of the WiiU and the power brick of the WiiU tablet controller would be the same in Japan and North America, but buying a WiiU without the tablet just to save money is foolish, because finding replacement parts would be a massive pain in the ass.
Even if you don't plan on buying many digital only games, definitely go with the 32GB model. 32GB isn't much space at all, especially considering the WiiU allocates a portion to its OS and updates. There are WiiU games and updates that exceed 8GB just by themselves. Don't shortchange yourself. SD cards are supported, but only for original Wii Virtual Console stuff since the Wii in the WiiU still has the pitiful 512 MB storage. External harddrives work for WiiU games, but accessing them is slow and Nintendo requires that external drives be formatted in FAT32 like it's 2001.
No product is 100% fail proof, but no, the WiiU is solidly build for reasonable use. Yeah, there are "reports" of people destroying the WiiU Gamepad tablet, but then again those are much the same people who managed to piece their TVs with the Wiimotes and cause injuries to those around them. The WiiU is not something you should be worried about failing or breaking as long as you don't do something like throw the controller across the room.
Depends on what you mean by "online support". All of the the original Wii's online functionality shut down several years ago save for the original Wii Shop channel, because that's a revenue stream for Nintendo and closing that down would be foolish. As for the longevity of individual game services on the WiiU, that should be longer than the original Wii because Nintendo essentially didn't have any idea what the hell they were doing with the original Wii. The WiiU is like the 3DS in that it has a dedicated network. How long will something like Mario Maker be supported for? Pure guess: but I'd say at least another 5 years. Maybe longer -- especially if the NX has some sort of backwards compatibility or at least cross connectivity.
The WiiU price will probably fall once the NX is released, but the WiiU isn't exactly the most expensive console on the market as it is. If you want one now, buy one now. Also, consider buying a refubished WiiU directly from Nintendo's online store. In Nintendo's case, "Refurbished" almost always means retrailer returned unsold merchandise. My "refubished" WiiU was brand new, or if it WAS used, there was no way to tell. Buying refublished saved me around $150.
Although the firmware technically supports it, the current firmware doesn't allow two WiiU gamepad tablets on the WiiU. Just one. For the second controller, buy a WiiU Pro controller. The controller is AWESOME and I greatly prefer it the gamepad. So buy two, even. Only certain WiiU multiplayer games support the Wiimote. Some do, but you're usually better off with the Pro Controller anyway. Most games don't require the use of the gamepad tablet at all. You can just use a Pro Controller instead. Depends on the game.
Not every game supports the WiiU's off tv play, but yes, generally it works for any game that just mirrors the display of the gamepad and tv. The resolution of the gamepad isn't as high as an HDTV though, so you generally dont want to do this unless you absolutely have to, even if its for playing WiiU in bed.
Range of the WiiU pad is about 12 feet. You can't go to the other end of the house and expect to use the WiiU tablet. You MIGHT be able to do it in an adjacent room, but that's about it.
I get about 5 hours out of the tablet. It's not great, but it's a huge deal to charge it. Just use the dock the deluxe version comes with when you're not using it. You'll rarely be using the WiiU for more than 2-3 hours at a time anyway.
Batteries are all the same size. Don't even pay this any mind.