Droideka are Ground vehicles so they cannot be engaged, correct?

If my opponent moves Obi wan into melee with my Droideka and attacks them, is he considered to be engaged? (Can I shoot at him with another unit?)

Then my Droideka can move backwards and shoot obi wan when they activate? (No withdraw is needed as they are not Engaged.)

Do I understand this correctly?

You are correct on all counts. Obi Wan and the Droidekas would be in melee (so they can use melee attacks), but not engaged (so no one needs to withdraw and both can be shot at).

Thank you!

The only disadvantage for you is that Obi might get cover from your Droidikas, when your other units try to shoot him. For everything else: Arnoldrew said it already