Anyone Got Any Ideas?

Everyone stay with me here. I
recently noticed a monumental error
in "Back to the Future Part Two" that
threatens to destroy everything we
thought we knew about the space
time continuum. So you know the
part when the doc is explaining to
Marty what happened to make Biff
rich?
He says that because Biff went back and
gave himself the Almanac in 1955, a
whole new timeline was created from
that moment. Now if old Biff had gone
back and given young Biff the Almanac,
as soon as he tried to go forward in time
again (which he did) he should have
ended up in the alternate 2015 instead
of the original 2015. Marty and the Doc
should never have gotten the time
machine back because old Biff would
have had to travel along the alternate
timeline never being able to get back to
the original 2015.
Biff was here
1955 Doc and Marty
Created this
timeline
Biff should in this
He should have gone to 1955, then
travelled to the alternate 2015 because
he gave Biff the Almanac and changed
the timeline. Any ideas?