Hi, boots with a small heel will be a lot safer than flats as they will stop your foot slipping through the stirrup. Personally trousers-wise I'd pick the gym trousers as I imagine they are stretchy without big seams? (I never set foot in a gym, can you tell? ). Lots of people ride perfectly well in jeans so you may be ok in those if they aren't too tight (you'll never get on!), especially as you will have long boots to protect the inside of your calf from getting pinched by the stirrup leathers. Definitely avoid the cords, they will rub I'm sure.
My hint for extra comfort would be some over-the-knee socks to wear over the top of your trousers to protect the insides of your knees. They won't take up much room in your bag either.
Enjoy the hen-do! And good luck with riding with a hangover .