C4 is on the regulated side of the low voltage: 10V is fine, the regulated voltage is 5V.
The reason for putting them in the board is very simple. If you have a failure, the WS2812B can be replaced without destroying the tube.
The trick is to take a small square file and very slightly file out the square hole if necessary. The problem is that the square holes are routed out with a round bit, and it leaves the hole a little tight. Two or three strokes in each corner of the hole is enough.
I see the results you have are perfect, but perhaps you had to squeeze the WS2812B a bit. There should be some spares just in case.