Is there a way to open Minecraft without using the launcher at all?

Not sure if this is what you're looking for but MultiMC is an open-source alternative to the normal Minecraft launcher. I've been using it for about a year and it works very nicely.

MultiMC has many additional config options & features over the normal Minecraft launcher, most notably the ability to run multiple versions of Minecraft with seperate .minecraft instances.

There are always other ways to do things in programming, so yes, there is certainly a way to open Minecraft without using it's launcher.

The thing with a launcher is, it does not slow the process of launching down, it only does the necessary steps to launch a game.

Not using the launcher and opening Minecraft with raw code won't make the process faster and might corrupt your game. It is not recommended.