Okay, there's at least one coda to the whole story.
Remember how I said that I retapped all the holes to practice? Well, there was a bit of swarf that came from each of them and each one wasn't exactly perfect. Well, once I tried to put the machine screws back into the threads, none of them fit anymore. I know that I used the right tap because the set came with a measuring tool.
Here's what I think happened: As near as I can tell, each of the screws are slightly bent. So I'm guessing that each of the holes were also slightly bent, too? Re-tapping them might have straightened out the holes and not the bent screws don't go in. I can also tell that this had leaked before (corrosion and over-torqueing marks on the screwhead) and I'm wondering if one of the POs knew something was amiss and put it all back - rather than trying to fix it.
The other thing it could be is that my tap and die set from the orange store could be slightly off? I tried re-tapping (re-dying?) the screws and it was taking off a lot of material. I thought that maybe they were metric screws for a minute, but none of the metric sizes were even close to the 5/16" 18tpi. I don't know - maybe there's a metric size that wasn't in the kit that's close.
Anyway, I won't know any more until I get new screws tomorrow.
No matter how long I own a boat, I'm going to keep underestimating how long it takes to do something. Who would have thought I'd have 10 hours into rebedding ONE handrail?
Mark