Be a little more specific - what do you mean by guru? Are you referencing Tibetan Buddhism? The Japanese sects? Chinese Buddhism? These things matter a lot.
Most of the Japanese sects for instance believe we live an age of Mappo, which bears some resemblance to the Hindu concept of lviing in the age of Kali Yuga…
for all intents and purposes - Enlightenment is “out of reach” until Maitreya Buddha shows up. Mind you, the Zen/Seon/Chan/Dhyana school(s) reject this notion, while the Pureland Buddhists have it as a built in given - which is why they have faith in Amithabha Buddha and keep chanting that mantra of theirs - in the hopes that they end up in his Pureland on their next incarnation so they can work on achieving Buddhahood in an environment where it is possible.
- Can one achieve enlightenment without meditation? For example, somebody would just be walking down the street and it would just “hit” them.
I’m not expert - i just have a lot of professional contacts - but i’ve never seen a Buddhist sect without some form of meditation. It seems like a built-in given.
- Can one in a state of enlightenment ever fall out of enlightenment and back to their “desires”?
Again - what do you mean but Enlightenment?
Do you mean reaching Buddhahood? - Then what you speak about according to them is Impossible.
Do you mean becoming an Arhant? - Yes, apparently Arhants can slip up.