Has the boat got a remoted sensed alternator regulator? that's the proper way to do it with split charging diodes.
The charging voltage of a battery is critical, for proper performance. The 0.7v voltage drops means the batteries aren't properly charged and won't reach their full potential, in terms of output, or longevity.
Yuh split charge diodes work, like you say, loads of people have and do use them. They're just a bit of a bodge in my opinion, especially when used without a remote sense alternator.
I agree with your first point - being a pedant, if the connection resistances were particularly high, moving the connection point could have an effect, but they shouldn't be, so connection position is, or at least should be, irrelevant.