Choose is seemingly unique because it ends in a /z/ sound despite being spelt with an s. If you look at the pattern I was describing, though, of oo-consonant-e almost invariably gives you a long 'u' sound, it'll be borne out. The only difference in pronunciation and spelling between 'choose' and 'chose' are the vowels, and in both words the vowels are acting entirely regularly. There aren't any words that end in /u:z/ that are spelt -oose, but that's entirely thanks to the /z/ sound. The spelling convention of oo-consonant-e is widespread and the exact same principle is being discussed upthread under the name "the magic e".
Because a mongoose isn't a kind of goose, so it follows the usual rule for plurals. As another example, "fireman" is a kind of man, so the plural is "firemen". On the other hand, "human" isn't a kind of man, so its plural is "humans".
