Soo, I've got a doubt; there is a portion of story when the kids start playing the game where Dave and Rose talk to each other and talk about how they survived while Jade and John died. Right after Dave went back in time and became Davesprite, then the place he came from disappeared; that place was the original timeline of the beginning of homestuck? Or a secondary, while the one with Davesprite is the original one?

That’s really debatable.  Davesprite’s timeline is necessary for the other one to occur, but the Alpha timeline cannot occur until after Davesprite’s timeline does.

Doomed timelines are generally considered to be offshoots with the Alpha being the original kinda like this:

But it is entirely possible that the doomed timeline is the original and when it’s travelled back in time and changed it creates another timeline which becomes the Alpha as so: