Fist of all, if you have a calculated field this field should be read only and you should not add a script to the exit event of a calculated field.
Considering that the Subtotal fields are already calculated (and read only) you can also add a script to the calculate event in the Total field to sum the Subtotals.
Answering your questions:
Q: Can a field have more than one event?
A: You can add scripts to different events in the same field, to enter and exit events, for example, since they make sense. If you have a calculated field it should be read only and should have a script only in the calculate event. In other words, a field can have more than one script in different events, depending on the situation.
Q: Is there a conflict between FormCalc and JavaScript?
A: You have to choose between FormCalc and JavaScript when you add a script to a specific event in the field, but you can use FormCalc for an event and JavaScript for another one in the same field. What is impossible is to use FormCalc and JavaScript in the same event.
Q: What would be a better way?
A: There isn't one answer. It depends on your knowledge, preferences and resources you want to use. I prefer to use JavaScript because you can do more things with it, such as to create functions, and because it is the same language (its core) used for Acro Forms and Acrobat automation. But some operations are easier to implement with FormCalc, such as totaling the values in a column in a table with a varying number or rows.