I would say the meter is suspect again.
only because while the numbers are different, the result is the same.
2 posts show battery voltage regardless of the number.
This means the VR should be seeing the correct voltage vs the dip you see without the bypass.

second
all those connectors are terrible. you can see some of them look burned. poor connections and not even water proofed. I would replace long sections of that wiring where you have multiple connectors on it.

third go to an actual store not harbor freight and pick up a reasonalbe multi meter.
I have a cheap 10 dollar one that works and even though I got the harbor freight ones cheap I don't use them for anything more than checking continuity.
or run to a pawn shop and spend a few bucks more on a fluke meter.

as for your voltage inside the van, that is 2 fold.
one with the lights on and idle you get lower voltage. it is a known thing and you see it start to go higher when you hit about 1500 rpm to 2k rpm your normal driving range.
two, that lighter voltage is going to be lower and an easy way to show the drop from the bulkhead connector.