I mained Cassiopeia in S5, but I still play her sometimes. I didn't like her rework, especially with the W. Doesn't synergize with her kit as well as her old one (although it is somewhat better in teamfights)
Personally, after Goddess Tear, I get a Ruby Crystal first and then build the other magic items to finish Rylais because she is so squishy. I typically don't build Zhonya on Cass unless the other team is all AD (which was your second game).
For the first game, it does seem that you needed magic resistance for the Brand. After completing Rylais, I would buy a null-magic mantle (to go into Banshees or GA later) and continue to my other mage items (unless she is really taking a lot of damage from Brand)- so basically buying the null-magic mantle and then prioritizing her damage items.
I don't think Deathcap should be your second item. It should have been Morello or Voidstaff. The CD from Morello is really important for your DPS on your Q. It also provides additional mana. Usually Morello and Void Staff are my 2nd and 3rd items (or 3rd/2nd, depending on how we're doing), and then Deathcap comes after that.
Otherwise, can't give you advice on how you played her as we can't see your techniques, etc.