Can I safely backwash from the port side water drain (at the Actuator Assembly) without forcing water backwards through the riser into the engine?

Did your impeller actually come apart while running? If not there is no reason to back flush.

Well, actually the impeller must of exploded nearly instantly when I started the engine with muff, while boat was on trailer.

Upon removal and disassembly of pump, there was absolutely no water in the raw sea water system (I had completely drain it last fall), and as far as I could tell, all (99%) of the impeller pieces were in the pump housing.

I want to backflush just to be certain there are no blockages.