Good skils with 3D printer. I have same exact part damaged, and will need to replace.
Regarding 2 gear.
When i was doing measurements how far each gear is, i found that second and fourth gear are only 1mm away. Is it possible that you do select 2 gear, only thinking it is 4 ?
Without metal plate that bolts on top is very hard to put into 2 gear, i would want to believe it's only that.
And don't just simply buy new gearbox, try to open it first, sometimes it is very easy fault
P.s thanks for gearbox support link, one is going my way from ebay