Well, by now I have completed all challenges in singleplayer mode as well. With that I pretty much cleared the entire game. I haven't collected all 15 Friendly Tokens yet, but I either want to do this with real people or in the new update, where you can buy them. I'll see about that, but I could get the rest of the tokens at any time, so this doesn't really matter right now.
Singleplayer went surprisingly well. It still would be nice, if you could steer the totem at any height (could be very useful for that spin attack throw trick) and if there was some way for the dopples to follow you Four Swords style. But you get used to stacking the other Links and carrying them around to save time. One main advantage of the singleplayer is the much lower damage most of the time. I felt like the majority of fights were a lot easier to handle, if you only offer one target, instead of three. On the other hand it's shame that you can only select one outfit for the green Link and the shared Energy Gauge can be very annoying.
While playing singleplayer I also took photos of all the 32 secrets from the Drabland Diaries (see Reddit for more info), which was a lot of fun to discover them all. You don't get anything for it and these photos are not really permanent, so I don't count this as part of the 100%, but I still did it anyway. That way the photo album at least has some purpose and now looks quite nice.