You need very little lemon juice (not even a tsp), if you want to make it separately and add it to the boil or to the primary.
Boiling wort in the BK will do the same (but the pH is higher and will likely take a little longer).
IIRC, the yeast can break down regular table sugar (sucrose) as is--it just takes a little more work.
Not a big deal. I've made invert sugar just to be able to say "I've done it."
However, I've not noticed a difference in the final product.