The linear pull should really stay the same, otherwise you would get really odd situations ex
Squid harpoons Galleon
The Galleon being huge barely moves
The Squid as the user is pulled less and barely moves
For the turning effect it seems plausible that the harpoon gun could launch the harpoon and then release the rope so it gets pulled from another part of the ship, i was thinking about having it centered on the side facing the same way as the gun but in line with the center of mass (for example a harpoon on a pyra would then pull from between whichever set of guns it was in) but that got complicated fast and would still leave several gun slots pointless for harpoon use (spire and mobula side guns).
The cargo hook in alliance mode attaches a rope to the center of mass of your ship which works well enough physics wise (though it doesn't have the extra pull force of the harpoon to compare exactly) and to me at least doesn't look too bad.