Niacinamide

Yes, twice a day if you like. It's one of the least demanding actives there is.

It reduces sebum production over time, so it helps with shine and with the pores that oil keeps stretching. It's not an instant mattifier.

Fine, and often helpful — it can reduce the irritation those cause. If you're new to both, introduce the retinol slowly and keep the niacinamide as the constant.

Usually the percentage. Drop to 4–5%, or check whether it's the alcohol or fragrance in that particular formula rather than the niacinamide itself.

Oil control in two to four weeks. Marks and tone, closer to eight to twelve. It's a slow, cumulative ingredient — which is also why it's easy to stay on.