Hi and welcome.

The instructions are by nature fairly generic, but I'd admit they could be better.

1. You don't have to adjust the curve of the neck yet, but when measuring things like scale length, you want the neck as straight as it can be. If you want to test for any proud frets before stringing the guitar, then you'll need to get the neck flat (but you'll need a notched straight edge as well as a straight edge to do it properly).

It's also good to test the truss rod works before you get too far. These double action truss rods have a slack spot in the middle of the adjustment where there's no positive or negative tension, and they can rattle. It's good to at least just get them biting with some resistance to stop any rattling.

2. There should be minimal room for movement with this sort of neck attachment. It should really be an interference fit. How much of a gap is there? If there's more than say 0.5mm, then I'd glue a piece of maple veneer to one side of the pocket. Which side depends on the neck alignment with the bridge, which ties in with item 3.

Obviously your bridge position is fairly well defined by the string-through holes, though there is often a small amount it can be angled by. But assume the bridge position is fixed.

You want to ensure the strings run down the neck parallel to the sides of the neck and end up at the right place at the bridge. You'd normally temporarily fix the bridge to the body (masking tape or double-sided tape), clamp the neck in position (you'll need a fairly large clap to do this), and run some thin string or cotton up from the bridge low E saddle, over the nut, through the low E tuner hole, out through the top E tuner hole, over the nut and back to the top E saddle. Tape the ends of the string/cotton in place if necessary so you can try moving the neck until the strings are aligned. This is where you want to have the neck positioned when you glue it. You may have ±0.5mm movement to move the bridge slightly sideways if it helps. If it does, mark the position.

This is where you determine if putting some veneer along the side of the pocket is necessary or not.

Also, now is a good time to test the fit of the neck pickup in the pocket whilst the neck is there. There have been some instances of the neck pickup not fitting properly. I believe the end of the neck should simply line up with the two open ends of the neck pocket, but IIRC, one or two necks had a different angle, so either left a gap or stuck into the pocket. Hopefully yours will be correct.

3. You should know exactly where you want the neck to sit. Apply glue (Original Titebond is recommended) to the neck heel and pocket sides where they'll make contact, slip the neck into place and clamp tight, using at least two clamps. The neck join length on this kit is quite long, so you should be able to get three clamps on.

Just ensure there's no finish at all on the surfaces to be glued. Always keep those masked. The glue binds to the cellulose in the wood. With finish on, that doesn't happen and the joint will be very weak. Wood to wood gives a very strong bond. And the surfaces need to be parallel so they touch, especially on the bottom of the neck as that's the surface that will be clamped and will have the strongest bond. Any big gaps need addressing as the glue doesn't deal with big gaps well.

You'll want some thin flat pieces of wood between the clamps and the body and neck to help distribute the load. Have the clamps ready and adjusted so they just fit on and need the minimum of tightening. The glue grabs quickly and it can be hard to make small adjustments once it does, so have everything ready so you can position it accurately and then clamp it in place.

Ideally you'd use a piece of wood with cut-outs for the frets on top of the neck. I bought some stick-on cork pads (meant for the back of coasters) and cut one of those in strips and stuck that to a block so they fitted between the frets.

Clamp firmly but not excessively tightly. You can squeeze out most of the glue if you over-tighten. Wipe away excess glue with a damp cloth.

Wait 24 hours. You don't need to wait any longer than that, there's no benefit. That's the Titebond web site guidance for joints that will be under tension.

Don't worry about it being strong enough. There's a big contact area. On the SG kits, the contact area is tiny, yet the necks are fine just help on by glue.

4. You check the scale length because these kits aren't perfect, so you want to make sure that holes have been drilled in the right place and all is in order before spending a lot of time and effort putting a nice finish on, only to discover that you can't intonate it. The kits aren't made on CNC machines, but by fairly inexperienced people using templates and routers and are prone to human error. This is the main reason you check things, ideally as soon as you get the kit.

5. Yes you can paint over the copper tape, though you'll want to run wires to allow the tape to be connected to ground before you paint over it. I prefer to use black shielding paint for exposed cavities as it's easier to apply and you shield and paint at the same time. But it can be messy stuff as the carbon in it comes off quite easily, so apply it as the last thing you do before installing the pickups. It too will need to be grounded, and I normally use a ground wire soldered to a small washer that I screw into the cavity out of the way (often beneath the pickup so it can't be seen).

Pickups are best left lose and just taped in position until you've strung up the guitar for the first time so you can align them with the strings before drilling the mounting screw holes. I'd always allow to use one set of strings for the set-up process as the could be on and off several times, and then put on a new set once you're happy.