can't write down 11 words?
I don't see any reason for not writing the 12th word. But if you have written down 11 words and don't have the 12th word for any reason, it can be easily brute-forced and there wouldn't be a big problem.
that really doesn't seem ideal. it makes me wonder about this whole checksum thing and if it's really all that important or just a gimmick.
Are you saying it's bad that there would be 128 possibilities for the last word? What's the problem?