The 56pF capacitor is a tuning capacitor to make the circuit resonate at the NFC frequency (which is 13.56MHz). See my thoughts at the bottom about this.

It's hard to answer this question without measuring it, which is what I'm trying to do. (I have 2 boards with different spacings and a SMA connector to measure the frequency response). But if I had to guess, I would say at 13.56MHz, you're looking at a pretty sensitive loop so imperfections in spacing would change your loop inductance and shift around the resonant frequency (which would change how far away you'd need to tap your phone). Are you manufacturing your own board or getting it made overseas?

You generally don't want 90 degree traces because this will cause sudden impedance changes and reflections at high frequencies. 45 degree traces also aren't recommended but I used them and it's been fine. Curved traces are the best!

Soldering this was super easy because I bought a stencil and there were only like 5 components to put down. I used an old hot plate to heat up the solder paste until it melted (but you can also use an old toaster oven). Let me know if you'd like to know more about this method.

What is the use of the 56pf capacitor in parallel with your 40mm antenna?

How exact does the antenna spacing need to be? I tried my best to get exact spacing but it's definitely tough and tedious.

Any difference between curved/angled and 90deg traces for the antenna?

How hard was the smd soldering of the NFC chip?