1 - Setting Default Location for every Transaction
The default location on a transaction for out-of-the-box behavior is from the employee record that's set on the transaction. You can also apply a Default Location for Sales Orders and Purchase Orders through the accounting settings: Setup > Accounting >Accounting Preferences > Order Management > Default Location for...
2 - Display subsidiary logo on printed transaction
If you want to display a subsidiary's logo on a printed transaction instead of the parent's logo, you can do so by going into Setup > Printing, Fax, & Email Preferences and deselecting the "location address" box.
3 - Country ISO Codes
While country ISO codes are NOT searchable in NetSuite, NetSuite does provide a list of the Country codes: