Gold Digger: Colour issue 50 has the (only) timeline in it. Here are a few important dates:
Gina is born in 1974, with Britanny born a year later.
Johnathan Diggers dies and becomes the Lich King in 1978.
Gold Digger Adventure takes place in 1987 (Gina and Brit are in junior high).
Gina and Brittany's first adventure where they meet Ace, as well as the Time Raft events, are in 1994.
"Hitchhiker's Guide to Lost Cities", meeting Stryp and Jetta, and defeating Gyphon, all happen in 1997.
The events of the black and white series begin in 1998 (so Brianna was "born" this year).
The "present" day (as in colour issue 100?) is 2004.
The event that gives us Ancient Gina happens in 2040. Though, something big is supposed to happen on Gina's 33rd birthday IIRC.
As to the third question, yes, Brianna does have some magical ability. It begins to manifest in colour issue 41. Not much is done with it by issue 100, but Fred might expand on it in the post-100 issues before the end of part 1. Or, it might be left until Gold Digger: Second Gen.
PS:
GD-ROM 2.0 is worth every penny. Much easier to find things (or just read your favourite parts) when all you need is a DVD instead of carting around a box of issues or the bricks.