In cryptocurrency, all blockchains I now of work like this:

• A private key is used to prove ownership of a certain address (public key) since the address is derived directly from the private key itself.

• You use the private key to "sign", or cryptographically prove ownership of an address, transactions that contain data about how much is being moved, where it's going, how much in fees you're paying, etc.

• This signed transaction is then broadcast to miners who then include them in blocks on the blockchain

Generating unsigned transactions requires access to the network in some way so that you can know how much funds are in the address and what the transaction nonce should be. Since these are just simple numbers, it's easy to just look them up on an online device like a phone and type them in to your offline system where you're creating the transaction. Alternatively, you can create the transaction online and move the data to your offline system and sign it there.

My point is, technically every blockchain can have transactions signed offline with the right knowledge. The question is which wallets support this so that average users can actually do it..

If you can't find offline functionality built in to the wallet for the blockchain you're using, you may very well have to get command line or API tools to do this.

You can absolutely add new wallets / software to the Tails drive after installation - just make sure it's in the persistent storage volume or it will be erased.

We'll have to see about how staking works on whatever blockchain in question. For Ethereum, it's based on transactions to the Casper smart contract, so you will need to be able to sign and broadcast transactions with some frequency in order to be an active staker. This is probably not the ideal application for air-gapped storage like the Tails drive.

To update something, you simply remove the old software / files and put the new ones on. In the case of MyEtherWallet, you're not really "installing" anything. You're literally just opening up an HTML file in a browser and using some scripts to handle stuff in an easy way.

If the blockchain itself doesn't fork to a new protocol, you should never need to update your software on an offline device.