How to recover lost partition after diskutil erasevolume noformat
The problem has finally been resolved... I went into Internet Recovery (but normal Recovery will do as well) and "reactive" the partition with Disk Utility. The old version of Disk Utility that is used when in Recovery mode allows to do this whereas the new version in System Preferences has pretty limited functions.

Basically, the partition had just disappeared before... When using the command sudo gpt -r show /dev/disk0 I had 4 partitions, but one without any identifier. As a result, I was unable to either mount or restore this lost partition with diskutil, as there was nothing to identify it. Sorry for not being any clearer, but since I restored everything, I don't have the output to demonstrate it anymore.

I still thank you for answering back!