Foreign Wiring & Electrolysis

Conventional thought is that AC current does not cause galvanic corrosion, only DC. The purpose of the galvanic isolator is to prevent DC current from coming aboard from DC leakage on another boat.
Rudder post corrosion may be poor bonding to the sacrificial anode bonding system or depleted oxygen corrosion. How was the damage discovered? Has the anode ever been completely wasted?
Isolated through hulls wasting are most likely poor bonding. If they're all discoloring to a greater or lesser extent, assuming the color is brightening of the copper in the bronze, then the difference in wastage among them could be wetted surface or proximity to the stray current source. Although if they are all affected, it indicates the bonding system is providing little protection. If they're coloring from bronze to darker, almost black, with some green, it's from moisture. What is the design of the bonding system? Is it transom or hull zincs, or shaft brushes and shaft zincs? If brushes, they should be metered and good contact verified often. If transom or hull zincs, they should be wasting at a rate sufficient to prevent growth attaching, if you can see them. Shaft zincs should require regular replacement, too. Zincs that don't go away aren't working.
None of the readings described or shown are unusual, or an "AHA!" moment, as regards corrosion, or even AC issues, but the AirCon hot connected to neutral and connecting to bonding could result in a swimmers electrocution if connecting to a solid ground when climbing out of the water. If the AirCon circuit was energized when hauled out, someone could have felt a tingle, or worse, when in contact with boat metals.
When tracing and diagnosing AC faults, it is best to use a True RMS meter, like the big brother to the one shown, here.

Are those photos from 3 years ago, or more recent?

Seasmaster,

In a simple 115VAC cord of European/Asian construction, brown is the hot (115VAC) and blue is the neutral. So I question the techs remark that brown and blue are neutral. Opinions vary, but on my 30 year old GRP (US operated, Taiwan built) boat with bronze thru-hulls, the bonding system is NOT connected to the AC electrical system (hot, neutral, or ground) anywhere and I do not have any thru-hull deterioration problems. My boat's bonding system is connected to the negative of the 12 VDC system.

You probably have a 115/230 VAC hot neutral, or ground conductor connected to the bonding system somewhere.

Assuming the dock pedestal is wired correctly and your dock cords are not modified, shut everything off and find the place where the shore power circuit from the dockside cord receptacle (on the boat) comes to your panel. Disconnect whatever you have to to isolate those 4 wires. They will be L1, L2, N, and G. Make note of the colors. You/your tech now know what voltages are seen incoming, for sure. There should be no connection between N and G on the boat while on dockside power - that connection is made at the dock/panel. The fractional AC voltage measured was probably correct. Do not start the generator. You now have to go through each and every piece of gear to insure that no electrical connections (L1, L2, or N) on the gear are mis-connected to G. The case of equipment is/may be connected to G. There should always be a high resistance (millions of ohms) between any circuit conductors (L1, L2, and N) and G when checking equipment on the boat (remember to unplug the dock cable while making these tests to eliminate the dock panel NG bond connection), and, IMHO, no connection (millions of ohms) between the bonding system and L1, L2, N, or G. You will have to check all cord/plug connected equipment on the boat, too.

Have you had any problems with the dock pedestal GFCI tripping at your home slip or when travelling?

European voltage standards are between 220 to 240 volts @ 50 Hertz. You need to check by country. All appliances from a toaster to a microwave are 220,230 or 240 volts. There is nothing lower.

The regulating bodies in the USA believe those voltages are unsafe for common appliances that use a standard plug and receptacle. There are exceptions, like a 240 volt window AC unit ,electric stove or dryer can have a dedicated outlet that matches the plug designed for that amperage.

PS: When working with either wires you don't know the color code or countries that don't follow any code, just use what's available. And you have many separate systems both AC & DC like on a boat, a Multimeter with "auto detect" just set to volts & it determines current type is really handy.