1) Rub some dirt on those gelcoat divots, you won't notice them from a
galloping horse. See
Problem Solving Matrix #1: Galloping Horse. Can you back up and take a photo of where that area is?
Those pores appear to be defects in the gelcoat, probably came from the factory that way and got bigger through the years. Maybe the mold was not cleaned properly, prepped with release agent or sprayed well. The hairline cracks come with age as gelcoat shrinks faster than the fiberglass underneath. You didn't create them, you just cleaned them out. Probe the area gently with a dull awl or small screwdriver to makes sure there are no soft spots in the fiberglass, I don't expect there would be as impact or stress damage usually has gelcoat cracks radiating out from a point. I'd fill them with something, as the gelcoat is primarily there to protect the fiberglass, from sun and long term water intrusion. Some folks go aggressive and take off an entire area of gelcoat, others would spray or brush on gelcoat and sand it down. Paint could be used or Marine Tex or a fairing compound, and if it were me I'd lightly skim it with a tube of Pettit EZFair and sand it flush.
2. Your Son is right, but on a small piece like an extension the weight gain of oak is negligible. Oak will last longer.
3. We have never thinned stain, but have applied additional coats to darken.
4. 2 is minimum, and usually maximum coats for us. That old daggerboard is probably pretty sealed up from years of varnishing. Go for the finish you like, and if someone else doesn't like it, they don't get to go on the boat. See Problem Solving Matrix #3:
If They Don't Like It
5. Yes on the thickened epoxy repair. Or grave in a new bit of wood and fasten it with thickened epoxy....or skip the wood...we're back full circle to thickened epoxy...you see how this boatbuilding decision making thing works?
6. OEM. Drift pin showed up at one point to help prevent grainwise fracture, mostly from folks letting the rudder slam up. A lot of folks pick on the old style rudder for popping loose but at least the blade didn't split chunks off... Hmmm, seems like I'm up too early...but I made a friend selling him a new rudder blade...
7. I'd scrape a small valley in that crack and inject some neat epoxy, shaken, not stirred into that crack, let it soak in, one side at a time. And I like the screw idea, similar to the drift pin. Pilot hole and countersink if you have em.
8. Rubber Baby Buggy Bumper...looks like a rub pad, or remnants of one, to keep the tiller extension machine screw end from scraping a half moon into the deck. One hack is to put the screw head down and nut on top, but if the nut falls off, away the screw goes to. Another hack, make sure the right size bolt is used so no extra threads are exposed.....make sure tiller is set to correct height...the list goes on...