Discussion Forum

http://blogs.magicjudges.org/rulestips/2016/08/brisela-and-commander/
gives the answer to that:
What if one of her constituent parts is my Commander? For example, what if Bruna or Gisela are your Commander, and you run the other half in your 99? Well, it’s very likely you’ll end up Melding them, and the combined Brisela will ‘count’ as your Commander for the purposes of Commander Damage, or for cards like Angelic Field Marshal and Bastion protector. If your Melded Commander is destroyed or exiled or any of the other zone-changes, and you elect to put it in the Command Zone instead, only the half that’s “normally” your Commander goes there.

As for your second question: No, a commander deck is always limited to exactly 100 cards per deck.