When Des was talking to Charlie he stated that his life flashed before his eyes. Was it all a "flash", a dream with the jewerly store owner being his conscience reminding him what has happened (and he sees the future because his life keeps on flashing on and on past the present) OR was it an alternate past that he was reliving?

I'm leaning toward alternate past because in the orginal events the bartender gets hit in the face (Des says so in the bar convo with his friend). In the "Flash" Des yells duck and the bartender does not get hit, Des does "waking" him up on the island. If Des doesn't yell duck thus not get hit, does he wake up on the island? Does this change the future?

So do we view this as a dream of past events, a life flashing before your eyes, or as an alternate past, living it a second time through?

I think the island was showing him his past to try and prove to him he can't change fate

I think he was reliving his past a second time, but chose to tell Charlie it was just his life flashing before his eyes because he didn't want to sound crazy. Crazier, I guess.