How loose is your cuff?
Sometimes my open-cuff crutches crash to the floor just like you describe, but usually they stay with my arm. The cuff is too tight for my forearm to slide out, but my wrist can still fit through the opening. Fortunately, I am now much more stable and my balance is a lot better than it was a year ago. Back then, if I needed to slip off one of my crutches to use my hand, I had to lean myself against a wall or something so I wouldn’t fall over and then lean the crutch against something, too. On stairs, I just go very slowly and carefully. If I need to carry anything, I always take a backpack. Basically, I try to be mindful of my situation and to plan in advance: I’ll take my bus pass out of my wallet and put it in a convenient pocket so I don’t have to struggle with it when the bus arrives.
If the cuffs on my crutches were too loose, though, I don’t know how I’d adjust them. The steel is really springy. Maybe you could stick some padding in the opening to make it smaller?