Lets give my smallest wheel and my longest seat post a go!
Starting with the only setup that lets my son reach any pedals with cranks longer than 45mm. 12"/89mm and the shortest seatpost cut even shorter:
This was rather tricky but unexpectedly doable. I have actually tried a 12" before on Jordalslekene but then the seat height was about twice the length.
So is it even possible for a 183 cm guy to ride the 700 mm seatpost without cutting it down?
Just barely, with the post rubbing the tire, on my toes and kinda jiggling from side to side, streching as much as I can! I guess 75 mm cranks would have been slightly more ridable (and made more sense I was struggling with pedal strikes when turning!), but I cut the post a bit nevertheless and put on a proper saddle.
So I gave this crazy 12"/89mm machine a 1 km spin yesterday and that was quite something. I was having a hard time not to let others simply walk past me. And when trying to go real fast, that is past 8 km/h or 140 RPM, I just had to jump off in the front since it felt so unhinged.
I don’t think I am even going to bother with 75 mm cranks on this nanomachine. I’ll be moving straight to 45 mm if they even work anymore. I guess I will try a 3 km ride, or maybe even 5 km like @Syoten (oh and did you give shimming your 50s another go, as discussed far above?), but furter than that can’t really be any fun at all. Speed bumps and curbs are challenging enough even on 89s. And 45s won’t really make matters better.