To simplify this expression, we'll first tackle the numerator and the denominator separately.

Numerator:
a-4 / a^3 - a ÷ (a - 1/2a^2 + 3a - 1/a^2 - 1)
= a-4 / a^3 - a ÷ (a - 1/2a^2 + 3a - 1/a^2 - 1)
= a-4 / a^3 - a ÷ (a + 3/2a - 1/a^2 - 1)

Now simplify the division:
= a-4 / a^3 - a ÷ (5/2a - 1/a^2 - 1)
= a-4 / a^3 - a ÷ (5/2a - 1/a^2 - 2/2)
= a-4 / a^3 - a ÷ (5/2a - 1/a^2 - 2/2a^2)
= a-4 / a^3 - a ÷ (5/2a - 3/a^2)

Denominator:
(n+2/n^2 - n - 6 - n/n^2 - 6n + 9) × (2n - 6)
= (n+2/n^2 - n - 6 - n/n^2 - 6n + 9) × (2n - 6)
= (n+2/n^2 - n - 6 - 1) × (2n - 6)
= (n+2/n^2 - 2n - 6) × (2n - 6)
= (n+2 - 2n - 6n^2 - 6) × (2n - 6)
= (n+2 - 2n - 6n^2 - 6) × (2n - 6)
= (-2n - 6n^2 - 4n - 12)

Now combine the simplified numerator and denominator:
(a-4) / (a^3 - a) ÷ (5/2a - 3/a^2) × (-2n - 6n^2 - 4n - 12)