As a family with a food allergy scout, it has been eye-opening to how little people actually know about food allergies; ourselves included. Until it came into our home, we had no idea how prevalent and how hard it is to find and identify food contents/allergens, etc.
To that end, I recommend incorporating your suggestion of pack & den level training on the topic. You can bring in guest speakers (allergists, professional chefs, school nutrition folks, nurses, etc.). You also want to incorporate the “sharing discussion”; scouts without allergies will often offer to share their candy, homemade items, etc. with their friends - so teaching the allergy Scouts to ask permission first (ask the adult who made it, ask a leader, etc. - because the scout offering may not know the ingredients or will guess) and teaching the other scouts to ask their parents to get/provide allergy free items to share. Same with parents; using store bought items that include the ingredients and allergen info is a safer way to provide snacks/food then home baked goods where an almond powder or nut paste may be used without thinking, etc. Most epi-pens include trainers, too, so a hands-on lesson on the epi-pens, how they work, etc. can be incorporated; including the misconception that epi-pens always work to stop a reaction, so it isn’t a big deal if the scout gets exposed, and that as soon as an epi-pen is used 911 should be called (there is a reason they are sold in packs of two).
We deal with Tree nuts and a severe Peanut allergy - many people don’t realize they are different; we have had friendly people swap peanut butter with hazelnut butter or almond paste and think they have addressed the peanut allergies. Many families tend to rely on pre-packaged snacks due to their ability to bulk purchase - but many of these products use peanuts; crackers with peanut butter, trail mixes with almonds and/or peanuts, etc. Almost everything these days has a warning that the food was processed on machines that may come in contact with nuts or peanuts, which makes it harder for people uninformed about the allergies to know what to use - since so many things appear to say not to use them. Being able to tell the difference from contains nuts and the legal warning that a nut may have been somewhere on the premises is important to talk about.
A couple other options to consider is go more pot-luck, with assigned courses to different groups instead of a pay-to-play method. We have vegetarians in our group, too. They typically will sign-up to bring an item that they can eat as the Scout staples of Hot Dogs and Hamburgers won’t work for them. So they have options because they team up and make sure there is a main dish and side or two that is agreeable (and allows those who don’t eat vegetarian a chance to try a new dish). The same can be done with your food allergy families; they can team up to provide a side or secondary main that meets the gluten, seafood, nut and peanut-free requirements. At the meal, having labels or areas of the serving table labeled as “contains peanuts” and “Nut/Peanut Free” helps.
Hope this helps!