The residue on your old scales looks like glue to me.
I like the Goop idea.Using sparingly. I think it works best if you use it like rubber cement and put a little on both the pins and in the holes (don't over do it) and let the Goop cure for 5-10 minutes, then mate the scales to the knife. Clamp tight and let cure for 24º. Goop has the advantage of being flexible. Super glue might work, but it may crack if the scale is smacked....say if the knife is dropped onto a hard surface. Superglue has nice tensile strength, but I don't think it has the same shear strength. The trick with goop will be to use it sparingly. I seem to get it everywhere when I use it.
It’s a pen knife, not the space shuttle. Superglue is fine.
I don't know what Goop is (perhaps something that Gwyneth Paltrow emits) but I was wondering if a silicon sealant would work as it is gluey but has a bit of body to it and is flexible.
As it happens, I'm a professional engineer by day, so the coefficient of thermal expansion argument struck a chord with me. Earlier today, I applied tiny dots of Goop to the scale receptacles, neatly assembled the knife, and put it in a soft-jawed vise. It turned out great and the new scales are rock solid! And the tiny amount of excess that squished out in one place was easily removed. Thanks all for the help with this project!