No, depending on how well the 3DS ends up selling it could do ~10 million but that is on the high side

12m is imo the maximum it could do. 9m is much more likely

It will not happen, higher sales than Wii are possible but even that is sort of doubtful. A large factor on whether the 3DS version outsells Brawl is dependant on what the 3DS version offers that the WiiU one does not. If it is a nearly identical title then it is doubtful that it will outsell Brawl, in fact if the 3DS version is a direct watered down 3D port then I could see it actually damaging WiiU sales.

Assuming WiiU sells up to 100 million units and 3DS 120 million then the games combined versions could potentially reach 20 million. But the 3DS version alone will not be able to reach that number. MarioKart has always been more successful then SmashBros and while MarioKart's popularity grew by leaps and bounds with DS and Wii, SmashBros didn't grow with the same ferver, leading me to highly doubt that SmashBros could replicate MarioKart DS's success even if 3DS sells as many hardware units!

The only problem is the lag or frame rate issues?  I assume it is lag which totally kills the game considering Smash needs to have 0 lag to feel right.  I only jumped online a few times on Brawl because of that.  Sadly, I don't have enough Smash friends like I did during my college days.  The only thing they need to address with online is to make sure everything moves the same as offline/local multiplayer.

Maybe just north of 10m, but no, not 20m.

And we can already assume that the 3DS version will be very similar to the Wii U version (character wise) because of conectivity. But I'm really hoping Nintendo nails online on both versions. If they can do this, then both versions should be a killer app, and the 3DS version could potentially be a system seller.

Oh the 3DS version will be a system seller even if Nintendo doesn't improve the game much from Brawl. Smash Bros on a handheld will push hardware regardless of the quality as long as it is within Nintendo's standards.

Personally while I am aware the games will be compatable I think Nintendo needs to offer something definative between the two games. I don't want to buy the same game twice, though I do want a handheld Smash Bros I really don't want to pay for the exact same title a second time. If Nintendo offers maybe two to five levels that are different between the two games, maybe gives the handheld version 2-3 exclusive characters and like wise the WiiU version.

Imagine you can play the 3DS version against a player on WiiU. It could be a lot like BattleField or Call Of Duty where some owners get special weapons due to pre-ordering or buying the gun online. Instead owners of the 3DS version could have access to two or three characters not found on WiiU and likewise with WiiU. This would encourage gamers to buy both games and depending on which characters are exclusive to which versions Nintendo could drive sales way higher.

Nintendo needs to offer something unique for the handheld version in order to ensure a ton of people buy it alongside the WiiU version.

No.
15 million if WiiU + 3DS is realistic.

combined with WiiU, of course it will, but alone, means WiiU version will sell nothing or that SSB fans will be 4xSSBB

I didn't find a way to say no.