I see you linked my post!
The only thing I have to add is on ProLogic II support in Wii Mode:
The only way you can use ProLogic II is if the Wii U is in Stereo Mode AND THERE IS AN IN-GAME OPTION FOR SURROUND. Most Nintendo published games (and many other games too) don't have an in-game option and rely on the setting you choose in the Wii's main menu. Since there is no way to select Surround sound on the Wii U without getting 5.1 LPCM, it is not possible to get ProLogic II sound on these games!
For example:
Mario Kart Wii uses the Wii's Main Menu audio setting (that doesn't exist on the Wii U), so you can only get stereo or LPCM 5.1 (which Wii games aren't encoded for) from the Wii U.
Resident Evil 4, however, has an in-game option to select ProLogic II. The Wii U must be set to Stereo mode first (to disable 5.1 LPCM), but ProLogic II does work in this game!
Note that the above examples apply to both HDMI and analog audio. Switching to analog won't fix the issue of only getting Stereo from many Wii games.