Ctrl-click/ctrl-right click - add/remove one part of the junction to the side chosen by player (similar to ctrl-click to build single belt). Removing part of junction should leave drug intact.
Hold ctrl to built stretch of belt in a different way (eg. first left then up, or first up then left).
Ctrl-right click on machine - sell machine w/o confirmation.
I believe ctrl is already used when building belts to enable the “manual” belt placement method (which used to be the only way to place belts). It would be nice to have more control over how belts place themselves, but you can actually get around that by using the manual belt placement whenever you want to do something that the automatic algorithm avoids.