Sorry it is late here so my answer may not be as in depth as you would like.
"Unless" is used when we want to put a condition on something, if people are fooling around in class a teacher can say "Don't come to class unless you are willing to listen" the condition is that the student has to pay attention or they will not be allowed to be in the class. Another example is "You cannot go on holiday unless you have the correct paperwork to get through security" the condition being that you have the correct paperwork.
Otherwise is related but used in a different way, for example if we use the classroom example again, the teacher can say "Pay attention otherwise i will kick you out of class" or the holiday example, "You must have the correct paperwork otherwise you will not be allowed past security". "Otherwise" is used to state the alternative whereas "Unless" is used to state a condition.
The only time I can really think of "Unless otherwise" being used together is when you are filling out documents where it usually says something like "Your visa will last for 2 months unless otherwise stated" which generally means that the visa will be valid for two months unless (condition) otherwise (alternative) is stated on your visa. So the visa will be valid for 2 months as long as there is not an alternative condition.
I hope this helps. :)