Have I erred in not performing istinja' for the light brownish discharge or slight drops of blood, not considered menstruation, before prayer, and in not inspecting my clothes before every prayer? And what is the ruling concerning my previous 'umrahs and my last Hajj if the brownish discharge was occurring and I did not perform istinja' from it?

You are not required to inspect your clothes before prayer to ensure that no discharge (kudrah) has exited, as the default assumption is that it has not. If an impurity, such as kudrah or blood, is found on the garment (and it is not merely a delusion), it must be removed unless it is a minor amount that is excused. You must perform istinja' (ritual cleansing) after the exit of any impurity. If you are unsure about the time of the kudrah's exit, it should be attributed to the most recent probable time of its occurrence. Your umrahs and Hajj are valid, and you are not required to do anything concerning them, as validity is the default. Beware of succumbing to obsessive whispers.