My chain rings need replacing but since I only use this mtb for commuting I thought I’d try and reduce from a triple to a double. Can I do this using my existing cranks? I’m already changing cassette and chain. Do I need to change anything else?

yes and no (Although for the latter some slick tyres with a bit of pressure in will make it faster/easier and maybe some guards too for the four rainy seasons we have in the Uk)

Yes just leave granny off but you need to adjust the limit screw to stop accidental shifts off the inside

If you’re planning on any serious making progress on the flat with 1.5″slicks it’s possibly worth bunging a slightly larger outer on.