Sealing isn't automatically required. If a hinge gap is slight, you really won't see much benefit. If the hinge gap is sloppy, you really need to seal. Experience will teach you what's tight enough and what's too wide. The gap basically reduces the effectiveness of the surface. Some gaps influence flutter. The ideal is to hinge the surface to almost touch and no extra work would be needed. As you do more hinging you work out the techniques to do that. BTW, there are a number of ways to seal the gap, with covering and with tapes etc. Search on "hinge gap". We got lots of information already posted and hashed out completely.

Removing the pins just before adding the CA is a good technique. Sometimes the pins cause a gap. Sometimes pressing the surface to close the hinge gap causes the pins to dent the balsa. They've served their purpose once the surfaces are pushed together, so they're not doing anything for you. As for ".. generously flexing the surface down (to drop in the CA)", don't get too generous. All you want to do is make it easy to get the CA in there. "Generous" may lever the hinge line into gapping. You're developing techniques here. Take a dry run or two to see how much flexing works so the CA applicator can reach.

The holes in the hinge slots is a debatable technique. The idea is to provide a sinkhole for the CA to use to get everywhere it needs to go. The slots themselves overlap the sides of the hinges don't they? CA is going to suck right down the sides with it's amazing penetration capability. I haven't drilled those holes after doing it the first time. And you especially don't need them when the hinge flats themselves have a slot that does exactly the same function. Save yourself some unnecessary work. Forget about those holes. But if you're going to do it, then drill them deeper than the slots in the hinge, because the slots are already providing a sinkhole twice as deep as your plan, but not all the way down.

Already hardened? Depends on the brand you use, it's age, the wood's absorption, atmospheric pressure temperature and how we hold our mouths. And whether or not somebody's talking to us while we're trying to get this simple task accomplished.

CA kicks off when the anti-catalyst in it gets separated enough from the two active components and those to have a clear shot at each other. We mix two part epoxy, and as soon as the two parts touch they start linking up. With CA, the two parts are mixed with a third. They're all together in the bottle and in the drops. When the CA starts thinning out, the two parts that lock on to each other have lots better opportunity to get together. So if the CA is still wet enough from the application on one side, that damp stuff hasn't kicked yet. And if it had kicked really quick, then good chance it didn't get completely around the other side. PH also deals with the anti-catalyst, but bottom line is, if the CA is still damp from adequate application, it's not kicked yet.

You may find that 3 drops on one side work great but that the other only needs 1. Sometimes you can tell by watching the absorption. The 2nd side really shouldn't need as much as the 1st.

BTW, you really don't need a lot of drops, honest. Some of those slots go completely through wing or stab or fin trailing edges. A couple of years ago, I autopsied a newbie's plane to show him how to look for why it crashed, and to teach him why it's a good idea to check ARF glue joints when you're assembling the suckers. I pulled the covering off the TE of the wing and it looked like a couple of bottles of CA in there. He'd wanted the hinges to not come out and had dropped 5-6 drops on each side. And he said a couple took of hinges "a lot of CA". He wasn't kidding.

The good ones learn from their experiences.