700willdo,
First of all, you can file for an extension of another 6 months after the first 6 months have expired while your mom stays in the US. But, it is not recommended that you do that if you want to keep her chances of reentering on the next trip high. Stepping into Canada and returning the next day is legal. However, I should have also pointed out that a continued pattern will lead to you being eventually denied entry into the US. Remember, the visa could be for 1,2,5,10 years but the officer at the port of entry decides whether you can enter and how long you can stay (which currently stands at 6 months). That said, if he feels that you are practically living in the US, he will in all probability deny you as it somewhat equates to abuse of the visa.
Helg & 700willdo,
I don't think there is any difference in applying for your spouse's visa along with you or after you. However, if the embassy in your country is weary or if your country is on the US 'watchlist' and you are ok with living apart for a few months, applying for your spouse's visa later is a risk-free option. They could reject you both if you apply together, however, once you are approved and living in the US they don't have a reason to keep your spouse away from you. If you do not belong to the country I described above, the main reason for denial that I can think of is finances. Get that right and you should be fine applying at the same time.
Hope that helps.