Does the supplicant bear the sin of not regularly attending prayers in the mosque after having duplicated the mosque's keys to enable prayer, and does attendance then become obligatory upon him?

You are not accountable for your absence from the mosque, and you may pray in any mosque you wish. You are not obligated towards this particular mosque or its congregation, because what you are doing is a voluntary act and cooperation in goodness. Nor are you accountable for the shortcomings of others, for Allah Almighty says: "And every soul earns not [anything] except for itself, and no bearer of burdens will bear the burden of another." Their reliance on you is a dereliction on their part for which they bear the responsibility.