Backward compatibility, version is another property, look at kis_pressure_rate_option.cpp : settings->setProperty("MyAttributeVersion","3.14")
If you implement paintLine, paintAt will not be called. If you look at KisPaintOp, you can see that paintAt() is called in paintLine() method.
Be wise and look at kis_debug.h, there you find some defined named outputs. You can use it like (similary to qDebug(), cout()): dbgPlugins() << "Hello" << QRect() << KisPaintInformation;
You can turn on and off the areas of debugging (plugins ,ui, core, ...) with kdebugdialog utility in KDE