Will this damage leather or factory paint on plastic trim?

The gel is formulated for multi-surface use on leather, vinyl, plastic, and fabric. Test on a small hidden area first if you're working with specialty finishes, but it's designed to clean safely across interior materials.

How do I apply this without getting it all over the place?

The sticky gel texture is the whole point — it clings where you put it and doesn't run or drip like liquid cleaners. Apply directly to the surface or use a brush to work it into vents and seams, then wipe clean.

One container really covers the whole interior?

Yes. One container has enough gel to detail a full interior without running dry. How far it goes depends on how much buildup you're cleaning, but it's designed as a complete interior detail solution.