I'm gonna throw this out there.....could be a bad prop....a bad casting ??
I look at those photo's and its not indicative of "pitting" to my eye...
props could be made from nickel aluminum bronze
or manganese bronze....so it's possible that materials in the casting are being " eaten"... maybe it's not bronze at all.
Prop could have been made in China
....always suspect..... ??
Prop cavitation can also make for little " explosion" on the prop surface, causing those holes.
Is the pitting noted only on the back of the blade ?? Any pitting noticed on the front of the blade ? See above for prop cavitation. If the front of the blade is " clean" likely a "cavitation" problem rather than an electrolysis
Is your boat " plugged in" while you are working on it.
What does the zinc look like after it's been on the shaft ? Is it still on the shaft. I note a zinc on the end of the prop, but are there others ?? Does the boat have a hull
zinc ?? How long does a zinc last on your boat when it's in the water
If you see electrolysis or pitting on the prop, you will likely also see it on other bronze thru-hulls. Do you ??
Many questions. Trying to resolve this on a cruising forum without being there is difficult.
I note that the prop appears to some kind of " folding" prop...I'm not familiar with 4 bladed folding props, so can't tell materials are used here.
But to your question, any stray current on your boat could find it's way to the prop.
Have you switched off all electrical
power prior to your test. Even switching off all power does not mean some or other wire inside the boat has not abraded and is touching something it shouldn't.
pumps are usually hard wired, check those.
Do your testing with the main electrical better switch " on" and " off"....
What about 110v wiring
Your shore power
cable could be in the water
at the dock
, causing problems.
Neighboring boats can cause a problem.
The fact that you are getting different readings as you disconnect grounding plates could also be a clue.
Yes, many questions.....trying to get you to think thru' some of these....