A waterjet cutting head assembly is provided which includes an orifice unit to generate a high-pressure waterjet, a nozzle body and a nozzle component coupled to the nozzle body with the orifice unit positioned therebetween. The nozzle component may include a waterjet passage, at least one jet alteration passage and at least one environment control passage. The jet alteration passage may intersect with the waterjet passage to enable selective alteration of the waterjet during operation via the introduction of a secondary fluid or application of a vacuum. The environment control passage may include one or more downstream portions aligned relative to the fluid jet passage so that gas passed through the environment control passage during operation is directed to impinge on an exposed surface of a workpiece at or adjacent to a location where the waterjet is cutting the workpiece. Other high-pressure waterjet cutting systems, components and related methods are also provided.