Will these cut stainless steel trim or only aluminum?

Tin snips cut sheet metal — aluminum, steel, and light stainless. Test on a scrap piece first if you're working with thick stainless; these are designed for standard trim and panel work, not heavy-gauge material.

Do the thumb locks stay engaged while cutting, or do I have to reset them?

The thumb lock holds the snips closed when not in use. Once you grip and open them to cut, normal hand pressure operates them like standard snips — the lock just prevents them from hanging open in your toolbox.

Can I use left-cut snips with my right hand, or do I need the specific angle for each cut direction?

Each angle is designed to work best in its intended direction — straight cuts for parallel lines, left snips for clockwise cuts, right snips for counterclockwise. You can force the wrong snip, but you'll get binding and rough edges. Use the right tool for the direction you're cutting.