Shotwell lists photos based on when they were taken. While this is fine for single-day "trips" (ie Saturday at the park) it is awefull for photos taken in a lengthy trips (ie My month in Italy) as each day would be an entry on the left panel instead a group. Having over 30K photos organized in directories, is there any way Shotwell will keep my photo structure and create one event per folder?
The simplest way to achieve this is with tags, not events. The short answer is to manually import folder-by-folder, and tag each batch of imports.
The first decision you have to make is whether to preserve your directory structure, and keep pictures in folders like you have them now. Shotwell defaults to a date directory structure, you have an event structure.
If you don't care, have all the photos outside /Pictures, and import one folder at a time using File/Import from folder... . Tag each batch as you go. All your photos will be reorganized in /Pictures based on dates.
If you want to keep the event structure, make sure to un-check the 'Watch library directory for new files' in Edit/Preferences/Library. Move all the folders with photos into /Pictures. Then import one at at a time, again tagging each batch of imports as you go. You will have to keep maintaining the library from then on, manually importing photos.