Sorry, Elizabeth, I'm afraid I'm not sure because I own a smartphone not an iPhone! I also regret to say that, for some reason, my MP3 player isn't working at all any more! I've decided not to use it anymore.
Yes I would think you can, its only electric you need to charge and you can buy usb plugs, but i returned our two as couldn't make them work, said they were loaded, could see the music, but wouldn't play. Got a refund.!!
To adjust the volume, I press the centre button - sometimes it takes me to the volume option, other times I wait a second or two and try again. Hope that helps. It's not a perfect device, but it holds it's power quite well, the radio is OK and my tunes entertain me on my long journeys across Britain and Europe.
I have returned mine for a refund, couldn't afford the time messing about with it. It played some songs and not others. Format error on virtually everything. Tried to update as some folks said even tried to restore to factory setting but couldn't. Sorry I am no help. Good luck