Is a person held accountable for what goes on in his mind, and can anyone other than God know what goes on in his thoughts, and can God forgive a person simply by his showing remorse?

Thoughts that cross the mind are forgiven unless acted upon or spoken. And regret is the greatest pillar of repentance. If a servant genuinely regrets, Allah accepts his repentance, and this necessitates the remaining pillars: abandoning the sin, resolving not to return to it, and rectifying any violations of rights.