1. Is it possible to have two different wallets on the same PC without going through too much trouble?
Yes you can! There is a parameter to the executable (probably also for the GUI
executeable) called "--data" with which you can specify where
to store all data (wallet AND blockchain) .. you will have the blockchain
stored on your disk twice then.
2. Is the .json file all I need to backup my wallet, along with the brain password?
Not the "config.json" you need to use the export feature and that file contains
all your private keys (one for the wallet and one for each account)
MAKE SURE TO REMEMBER YOUR PASSPHRASE!! private keys are encrypted in the backup file!!!!
3. What percent of your holdings (btsx and other crypto) are on various exchanges?
0.00000% expected more?
Currently I have like 80% store in coldstorage (see wiki.bitshares.org)