The main food allergens are all due to the unique protein content they contain with the major exception being sulfites (sometimes added to wine and other foods as a preservative). The eight main food allergens are milk, eggs, soy wheat, fish, shellfish, peanuts & other nuts, and typically if a food is served where the protein content of these foods has been substantially removed, the allergy will be gone or not be as severe (eg butter from milk mostly milkfat). This may explain your reaction to nuts & avocado, but cantaloupe melons and most fruits & vegetables are typically very low protein content (cantaloupe has about 0.8% protein) since these are made mostly of water (~80-95%). basic sugars (glucose, fructose, sucrose), and insoluble & soluble fiber (cellulose, pectin, etc.). This does not mean you are not allergic to cantaloupe, only it is more rare, and likely not related to an allergy of nuts.
When the nuts are roasted and your symptoms seem less severe, this could be because the heat treatment when roasting the nuts may denature or partially denature the proteins changing their conformation and making them less reactive since binding sites may no longer 'fit'.
Another possibility is that it may not be an allergy per se, but the unique microflora you have in your intestines are going crazy with these certain foods you eat and producing excessive gas from the 'fermentation' and this is causing the pain due to high gas pressure causing bloating and discomfort. This is what is happening for people who are lactose intolerant and consume milk products. Since they lack the lactase enzyme to break lactose into glucose and galactose to be absorbed, it gets passed through and gut bacteria then 'ferment' it and the gas byproducts cause intestinal pain and possibly diarrhea.