Backstay lower end should be insulated from chain plate...you want the segment between there and the mast head end to be the radiator -separate, electrically from the rest of the rigging, coax ground should then clamp/attach to the short stub that would attach to the chain plate.
Ground straps should show little to no resistance when a DMM is used, between those straps and other boat grounds. Many use a ground plate on the exterior of the hull, others may use a large metallic mass/plate/ground plane as low in the bilge as is practical. Some will use the engine, or prop shaft as "ground". You want ALL of that ground to be at the same potential (no voltage between the various grounded bits and plates and straps). Similarly all connections should be clean, tight and and zero resistance between those connections and equipment ground studs or connections.
The dummy load will eliminate the rig (and the tuner, on the next step, test) as being faulty, if you can see a dramatic increase in current as you transmit, briefly. Then add the tuner in, and retransmit in to the dummy load, you should still see a current increase as you transmit and a quick reduction when you release the microphone. Make sure your dummy load will handle the power, within the timeframe you need....most are rated for seconds.
Similarly, you should see a reduction in Voltage at the battery/source as that current rises. Perhaps 13.6vDC dropping to 11.8-12.5vDC...if the battery/source drops much below that, you may have a weak/dead cell or bad battery. You could check this in your current configuration...just place your DMM across the battery/source and see what voltage drop you see on transmit.