You can fix it and re-submit now. You need to email them first but people do it all the time. I'll try and find the email. It's ted something at ATF. They will turn it right around for you. Don't give up
As far as the Multi thing. Just don't put multi in the description and when you engrave your trust just add Cal. 5.56 (or what ever cal you are using) I've done it many times on lowers marked from the factory as multi. They ATF doesn't like that word so don't put it anywhere on the forum.
Edit: Here is the info you need to push it through now, or you can give up?
Submit a new application send an email to Ted.clutter@atf.gov and include the control number of the rejected application and the control number of the new corrected eForm and the application will be reprocessed