HI
since i just tried sunless tanner for the first time last week, your post caught my eye. I had a similar dilemma, except my tops of feet and inner wrists were streaked. what was worse, it got darker and more noticeable 2-3 days later. i wore long sleeves all week despite the heat. there is only 2 things i can think to try, one, which is what i did, was take a shower and get a good loofah scrub sponge(rough surface) and scrubbed my arms and feet. since the nature of sunless tanner is to bind with your skin cells at surface, and it fades when your skin cells die and fall off. it did seem to work, it was gone. however, i did that after the stuff had already been on about 3 days. also my skin was left a bit red for a few hours. another thing you could do is get some advice from someone who know how to use make up real good and get some foundation and concealer and powder and you could probably work with it and conceal it nicely. and at least that washes off!
i would not recommend adding more tanner you could make it way worse. i was afraid to touch the stuff for a week after. you may want to wear a choker necklace or something to take away from the spotted look. the only other thing that may blend it is going to a tanning salon and trying to get a nice even tan(or burn) which i guess defeats the purpose of using the stuff to begin with. i hope someone else has a better suggestion.
good luck and try not to panic. it is usually dim lighting at proms and everyone will not be focusing on your neck. just keep telling yourself it will fade in 3-5 days.