What are the parameters for permissible speech and speech for which the speaker might be sinful, especially concerning everyday ordinary conversations?

The obsessive whispering (waswas) has reached an extreme degree in your case because you have indulged in it and surrendered to it. The best cure for obsessive whispers, after seeking Allah's help, is to turn away from them. Permissible speech, such as asking about people's well-being or the weather, is harmless, and there is no need to scrutinize or delve into it. One of the most beneficial remedies for obsessive whispering is to turn to the remembrance of Allah and do it frequently, and to feel joy when you sense these whispers, because Satan hates the believer's happiness. The effective medicine for obsessive whispering is to completely ignore it. Whoever listens to it will find it increasing until it drives them to the brink of madness. Obsessive whispering comes from Satan, and its cure is to believe that it is a satanic thought and to fight Iblis. Thus, the one afflicted with obsessive whispers will have the reward of a mujahid (one who strives in Allah's cause).