Similar to bitcoin miners, a validator processes new ledgers (blocks) whereas a watcher just keeps track of what's happening in the network.
However to participate in consensus you'll also need to be trusted by other validators, so as long as nobody includes you in their quorum set, there is basically no difference between a watcher and a basic validator ...
First, you need to decide how to defined the price of a coin, should it be:
The price of the most recent trade?
The average price of the most recent n trades?
A volume-weighted price of the most recent X units traded?
The midpoint between the current bid/ask?
You can get the data you need from Horizon:
Get the most recent n trades for a coin: example
It's a complete reset, starting over with a brand new ledger from zero. Your dump/backup will not be consistent with the new network state by any means and it won't work at all.
You have to create new test accounts and it is best practice to automate that.
Other than that you may run your own private testnet or standalone node and thus be independent of ...