As a Mustang owner, I've learned a couple of things. One: The interiors produced before 2015 were total garbage. The plastic is tacky, the cloth rips and other components tend to wear down very easily. HOWEVER, I do find that the leather conditioner does a surprising wonder on the faux leather and plastic material. Creates a clean look! Two: If you have expectations of having a car without issues (in terms of Ford products), get rid of them. Ive had one hood with rust issues and another one thats starting the process all over again. Also having some clear coat issues on other areas even though they've been protected and such.
However, even though theres some issues, I still love the car to death! To answer your question of cleaning your interior (keep in mind that I do NOT have a premium interior package), I use waterless wash to get any dirt/ residue off of the plastic and faux plastic, wipe to dry etc. When applying the conditioner, I use very small amounts and spread the conditioner on the red application pad with my hand (using gloves). This would properly spread the conditioner on the surface in which you would like it to be applied to. It would also get rid of your smudge issue. In terms of amount used per panel, my best example would be the size of a nickel for the first half of the door that includes plastic and faux leather. I'd suggest more for the other parts, should come out ok after! Hope this helps!