Is a man's solitary prayer sufficient to be a pillar of faith, and will it be accepted from him, and to what extent will it be accepted?

Your prayer offered individually at home is valid and does not need to be repeated. However, you have missed out on much good and committed an abandonment of an obligation, as congregational prayer is obligatory according to many scholars. As for its acceptance by Allah Almighty, that is an unseen matter. Therefore, a Muslim must exert effort to ensure their deed is accepted by performing it in the prescribed manner, with proficiency and sincerity. The pious predecessors used to fear that their deeds would not be accepted.