Even with perfect measurements it’s impossible to get to that line straight unless the frame is perfectly square. This not only completely solves that issue but also saves a ton of time not having to measure and cut them one by one.
Also the lumber I picked turned out to be 10 cm longer than the deck and I even picked the exact right amount. By the time I was done I only had a basket of <10 cm off-cuts.
Not really a problem. The wood will dry out regularly, so it won’t rot.
End cuts trapped behind fascia can retain water if they’re not completely sealed off from it getting in there in the first place. Better to treat your ends before covering them because even if you do it perfectly now, the deck can get worn or damaged later and allow water intrusion. Lasting builds design for reasonable fault tolerances.