Given his role as a tragic hero in the novel, is it inevitable the Okonkwo commit suicide?

Yes, Okonkwo's humiliation when he loses his place in the community, and his ultimate decision to take his own life rather than surrender it to the "weak" white man dictate his suicide.

"Okonkwo's suicide seems nearly inevitable. Determined to fight the white man, alone if necessary, the betrayal by his people is more than he can bear. He realizes that he will resist alone, even after the outrage of the white man ordering a stop to the clan meeting. Okonkwo understands that his people have been broken. Instead of a war, he will have only the white man's noose; he will not even be tried under his own people's laws. He chooses suicide instead.

Long years of difficulty and disappointment have contributed to this moment. The accidental death and then exile darkened Okonkwo's view of life. The betrayal of his son was a very heavy blow. Now, the betrayal of his people, and their inevitable subjugation, pushes Okonkwo into despair. Okonkwo's central beliefs have been undermined. He believed that a man was the master of his own fate; his exile and the loss of his son challenged that belief. He also had great faith in his clan, but now his clan will be a subservient people. He cannot bear this disgrace. Parallel to Okonkwo's tragedy is the tragedy of his people's subjugation. As a final bit of bitter irony, Okonkwo's suicide violates the very traditions that are being menaced by the white man."