So newb question here, I have the trezor and the ledger and myetherwallet.com

Im a bit confused about what is needed to get into your wallet?

1. Is the "back up wallet" a sensitive file? can i just back it up on my dropbox and a few other places so I dont lose it, or does it have to be somewhere offline?

2. Do you have to write down your private keys? will you ever need it?  Can someone get into your wallet with the private keys?

3. Can someone re create your wallet with the master 24 words password alone?

thanks!!! super newb stuff sorry lol