If your having bridesmaids and groomsmen, give them a color palette/style and have them go from there. I'm thinking May is the perfect season for something like this.

A wedding dress is what you decide it is - maybe there's some heavily discounted prom dresses you think are beautiful, go with that! May is the beginning of wedding season, so don't be afraid to get creative.

Also, look in to a sound system, especially for an outdoor wedding. Look into rental places or DJ companies for help with that. Even if you have a small wedding, if there's too much wind no one will be able to hear you say 'I Do'.

If you have the ability, try to spring for a photographer. If you have a friend who is learning how to use their Canon Rebel, that works! Otherwise, try contacting a local college/high school and see if anyone there is trying to start their own photography business. It may seem like a drain on funds, but a couple hundred dollars to have a photo of you, your FH, and your grandmother will be sooooo worth it in the future. Especially if you do a vow renewal 5-10 years down the line, you can feature those photos like a before and after!

Love the idea of a pot luck! I'm assuming a lot of your family and friends are familiar with the situation, and understand the low budget and urgency, I'm sure they will step up.

Final note: You can do this! My cousin got married in three months, and the wedding was just as beautiful as if were planned in a year. Just don't be afraid to take the help of family and friends when they offer.