1. If I delete the files that are currently on LRCC as trial, will the corresponding files in Classic remain?
2. After migration to LRCC should Sync in LR Classic remain off, if so, will changes made in LRCC be available on the desktop in LR Classic and vice versa?
3. Should all images imported into the LR ecosystem be done in LRCC and will they sync to Classic?
Lightroom CC desktop works just like a mobile app you are using now. Whatever the change you do via Lightroom CC desktop the changes will be available in Lightroom mobile & Lightroom Classic CC version.
If you haven't migrated from Lightroom Classic CC to Lightroom CC, you can access all the images in Lightroom CC what you have in Lightroom mobile.
Refer the below article for more information.