2) Why would Sigma make a lens that targets only Nikon users, while it perfectly knows that more than half of DSLR market are Canon users?
Now that's a valid question. But it looks like that is what they have done. Unless in the field the word filters out that it has some significant advantage over the Canon such as sharpness, bokeh, colour etc. But I can't see it.
I do think that Sigma DID initially plan to turn this lens into a head-to-head competition against Canon's 24-105 (not as a strange product that may or may not target only Nikon users).
They just were too slow.
I do not know what time it takes to design, test and put a lens into mass production. But probably it takes few months (or rather -- more than a year).
The most drastic drop in price for 'white boxed' 24-105 L happened this year (too many unkitted 6Ds and 5D3s sold to those who already own 24-105 or do not want one) and 24-105 L price suddenly dropped to just $600-700.
So, one or two years ago Sigma might do have reasonable plans to make a fortune selling its own version of 24-105 for $850-900.
Now they can't. But it's too late. They could either kill the project (that's already finished) or proceed with it as it was planned...