My experience with food restrictions is that if the hosts ask, you can tell them. If the hosts don't ask, you eat ahead of time/stick something in your purse and eat what you can from the meal or go hungry.
If the hosts don't ask, you don't phone them up and say "I'm coming to your wedding. By the way, I need a gluten free vegetarian meal, and my husband gags at the taste of fish" (or scribble the same in your RSVP) because it puts pressure on the hosts to accommodate all such requests, whether or not it's feasible or affordable.
As an aside, it occurs to me that the alternate drop is particularly unfair to people attending the event alone. If I can't eat my dish, my spouse would swap with me even if they preferred the one they had. If one dish is better than the other, we can always share both dishes and even it out. But if I'm alone and I get the inferior dish, I'm probably stuck with it.