Re: Who would validate transactions if all miners shut down?
Please, spare me, dont give me answers like "it will never happen" 
All nodes validate transactions, even those who don't mine. So I assume you mean who will put transactions into blocks? Mining IS building the blockchain. No miners = no blocks = no confirmations (which isn't the same thing as validation) for transactions.
i guess you could run poolers cpuminer in the background if there was an apocalypse and you were the only 2 people on earth