Once the download is on the tablet can it then be cast to the tv via Chromecast?
I don't think so (or at least not without streaming it again).
From how I understand it, Chromecast isn't just relaying tablet contents to the tv, it's streaming it separately, and the tablet/phone is just being used as a remote.
Someone else may understand better though.
I checked this morning, and some of my kids favourite shows are available for download, so this will be a lifesaver when we're out and need to keep them occupied. I burn through so much data when I have to stream YouTube for them.