My FI has been married before and this (justifiably as the history books found out!) was what transpired before he and his ex got married. In the end he turned round to his parents and bluntly pointed out that the more interfering they were, the more determined he was to marry her. He also gave them the ultimatum that if they could not be happy for them then they need not be there to witness their happy event. His parents very quickly pulled themselves back into place and the day went fairly smoothly.
I guess in hindsight, their objections were well founded. Fortunatly we all get on like a house on fire, I'm not sure I'd like to see my FMIL cross with me!
I know it's stressful. I dont know the legalities, but maybe you could have a quick registry office wedding before your DW - that way if they do try to pull anything it doesnt matter as your already husband and wife? Stay strong, and make sure that your FI sticks by you. The day is about you and him. Screw everyone else sweetheart!