Why do you have to tear the boat apart to add a different set of mounts? Marquipt tells me they are just screwed into the hullside with the specified size/length screw and a little sealant - no backer plates or bolts/nuts/washers in a solid glass application, unless you just want to through-bolt them, which yes, is always better. The hullside mounts don't have the "pulling out" forces on them. The force is up and down and the number of screws used to install them will keep them where they belong. If you're balsa cored at that spot, then you'll need to through bolt them, but I don't think any of us here have balsa below the rubrail, do we?
Anyway...I waited a long time to find mine because I wasn't willing to fork over thousands of dollars for them. I had steps, they were just cruddy and needed the bottom step replaced, but still serviceable, so there was no real rush. I searched my local Craigslist every day, sometime several times a day, using the term "Marquipt" and at long last I landed a newer style set with two offset handrails topped with a starboard grabrail, for just $500. They started out listing for around $1,300, but I watched and waited until I got them for $500. There are three sets on South Florida craigslist right now, ranging in price from $700 to $2,450.
Oh...just remembered...if you found a set you like with the newer style mounting, those tongue & groove mounts are removable from your stairs - take those off and they become the old style mount that need the pin. That's what I've done with mine until I get the new hullside brackets.