What do you do if the "first" word (of 12), happens to be the last word in the list alphabetically?
That couldn't happen. If one word is the very last from the wordlist, it would end up at the end of your mnemonic once you rearrange your 12 words alphabetically.
However!
(@vjudeu) Choosing 11 random words and then sorting them alphabetically before assigning a checksum would reduce entropy considerably. If you think about it, to bruteforce the entire keyspace one would only need to come up with every possible combination of 11 words + 1 checksum. I'm not the best at napkin math, but I think that leaves you with around 10 trillion combinations, which would only take a couple months to exhaust with hardware that can do 1 million guesses per second.