Can I legally use the TM symbol before I apply for a trademark registration? Yes. But not the ® (circle r) symbol. Learn to use trademark symbols properly.
Yes. Displaying the registered trademark symbol ® provides actual notice and provides the ability to claim profits and damages in a later trademark enforcement lawsuit.
No. You are not required to hire an attorney. The US Trademark Office says “A private trademark attorney (not associated with the USPTO) may help you avoid many potential pitfalls.”