Sometimes Dan can't shoot to distance, it's blocked in ground or something but if jump bullets go away

I've beaten the game (absolutely loved it its brilliant), and there's one door listed as door? that seems to be unused for regular playthrough with a long numeric passcode. Has anyone found any leads on how to access this yet?

Anyone else experiencing frame rate in the low 40's during certain sections of the game. GPU is showing as hardly being taxed and its running off of the internal memory so believe that rules those two potential issues out. Cannot understand why this would struggle as the game in some areas is fine and the graphics arent high res....

After my second playthrough of the game, I come back at Chapter 19, collected the last 4/5 LuxoMail that I was missing and reached the door, but Ben didn't have any prompt to interact with it. I tried to move on Chapter 20 but during the final run I didn't encounter the door anymore. Did I perform a wrong route or I missed it? Do I have to complete this achievement in Cpater 19 or 20?...

How hard is the platforming? I do not play platformers often. :(