Thanks for the comments Evan ! funny you ask about calculating returns, another community member, colvano, has put together a calculator to figure out exactly what you are looking for. :-)
How much will I earn from staking? How much NAV Coin do I need to make a profit? Calculate how much NAV Coin you need…navtechservers.com
As for safety using the wallet in an public network… there is none really, whether you’re staking or not. Not saying the chances are high of getting ‘hacked’ by doing this, but it certainly isn’t advisable. If you don’t own the router, who knows what is going on behind the scenes. If a public network is your only option, I would suggest using lite version of the wallet, you wont get staking rewards, but will know your coins are safe. There is an upcoming release of the NAVpay wallet which is being coded in house which I would highly suggest you use. There is currently a lite version which was forked from coinomi, but the upcoming NAVpay will be much better! good luck.
All transactions being verified by the wallets are indeed only NAV Coin transactions.