Hi Dan,
1) Answering the question you asked in the topic, yes you don't have to use load-balancing if you don't desire. Load balancing lets users create rules that divide traffic between different interfaces
2) No, if you don't use load balancing you will still get the speed of your chosen primary connection.
3) I'm not sure why you would want to do that but no you can't use your mobile phone as your 3rd WAN