test="B/following-sibling::C"

does what you want, in 1 or 2.

in 2 
node()[self::B]/position() < node()[self::C]/position()

is legal but


node()[self::B]

equivalent to

B

so

node()[self::B]/position()

is B/position()

which selects a sequence of B nodes then for each returns the
position,so in general it returns a sequence (1,2,3,...count(B))
for your input it just returns
1

node()[self::C]/position()
is also 1
so
node()[self::B]/position() < node()[self::C]/position()
is
1 lt 1
which os false()

David