Witnesses are basically miners. Ned computed the number of nodes needed to make sure that the data stayed solid, and that is how many witnesses are paid.
You can write a wallet if you want to. There is all kinds of libraries for accessing the graphene block chain. However, it would be almost exactly like the wallet page on steemit.com. No more and no less secure.
Your master key is the only security in the entire system.
Yes but I could not find any software on the steem.io website.
I think there is a Github repo, have to look at it.
Although I am sure there isn't a specific LInux wallet out there , as lightweight SPV wallet like Electrum/EC, so the only way to access money is from Steemit or Busy i think.
There is a Desktop Wallet developed by @jesta named Vessel. With that, you can manage your account keys and keep them offline but even if you don't trust the online or desktop wallet there is still the possibility for a paper wallet.
https://steemit.com/vessel/@jesta/vessel-0-2-0-interact-with-steem-securely-from-any-website