2) When it comes to the setback, what matters is that we can achieve our preferred saddle position relative to the BB. This in turn can be determined from just 2 factors: The seat tube angle and the setback relative to the line through the bottom bracket. The reason the setback looks big is that the seat tube is shifted forward relative to the BB as seen by the blue line.
i) My current bike has a STA of 73.5 instead of 73 on the Graro. That results in shifting the saddle back roughly 5mm at my saddle height, relative to my current bike.
ii) My current bike has a setback of maybe 5mm, (round seatpost centered on the BB). This means another net 15-5 mm = 10mm setback.
iii) However, the clamp that holds the saddle rails is also much smaller on the Graro, which gives a bit of leeway the other way (roughly 7mm by my estimation).
I've concluded that I can easily achieve my current saddle position, even without slamming the saddle all the way forward.
Yes, that is indeed the way it should be done. With a detailed geometry compare site you can see if the setback will work for you.
A tip: not all saddle rails are equally long, so if you can't find your correct position with the saddle you have, you can always see if there are saddles with longer rails. With my Hygge (only available with setback seatpost) I can only use one particular saddle because the rails are longer in the back. Nearly all the carbon saddles have too short rails in the back.
I should have done more comparing between frames and bought an other frame with less setback
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