Part 1: If someone is speaking in a teaching role and blatant heresy is detected that the pastor and deacons missed or overlooked, what should be done after the initial confrontation of the speaker, if their response is "oh well"?
Part 2: If the leadership*makes excuses to downplay the heresy, should you confront the leaders or leave the congregation?
One of the advantages of a confessional church is that much of the essential doctrine is written down and the leaders are accountable for it. In theory, when the congregation confirms its leaders (presbyterian system), one knows the essential theology of the leader.
In your first case, I'm thinking a congregation member biblically goes:
first (humbly) to the person
if he will not hear,
second (humbly) to a member of Session
if they will not hear,
third (respectfully) file complaint to Session as a whole
if they will not hear,
forth (respectfully) file complaint to Presbytery
there is even a process in which if a presbytery fails to act, two other presbyteries can petition general assembly to assume original jurisdiction and they must do so.
While no system is perfect, there is every opportunity to address serious doctrinal error, biblically in this kind of process.
It's also helpful that in our denomination some of the more sophisticated serious error going around (e.g. "federal vision") has a helpful denomination study paper with 9 points that can be used by individuals, sessions and presbyteries in ferreting it out if it pops up.