You forget Time Traveler,that in the animated movie "Yellow Submarine",Jeremy The Nowhere Man makes roses bloom all over the leader of the Blue Meanies!
I can only assume you watched that film quite a bit. That particlar part showing up in your dream might mean it's a good memory to you,in relation to John & the Beatles.
The Blue Meanie could be Mark Chapman crossing paths with Lennon. Meaning the roses coming out of Lennon's coat might represent the moment Chapman says "Mr. Lennon?" and "so forth".
When the man & woman (or John & Yoko) sit on the ground it could be an interpretive move meaning she's going to be puttinng him to rest there (wherever that ground is) and what's more,that ground could be Central park in NY where they held the vigil.
The before hand part about the tunnel,could be that you connected yourself so closely with John/Beatles that (in a universally connected way) you predicted the worst. Going into a dim or dark place in a dream is almost never a good thing.
I hope that helps you understnad why you had that dream. I know in my own experience,there's dreams I'll never forget.