The warning about getting the SSN before marrying is if the marraige certificate shows a name change for by the k1 holder. Unless you are changing your last name after marrying your fiancé, there is not an issue for your SSN application
Go to SSN asap.. nothing to do with you getting married. Breathe 😁 File the AOS as soon as have the paperwork in hand.
The K1 “ expired “ the moment you entered the US. Your period of authorized stay ends 90 days after entering. Getting a SSN when entering on the K1 must be done well before the 90 day ends otherwise the SSA check on your lawful stay Status won’t get completed before the 90 days is up. Once the 90 days ends, you cannot apply for a SSN using AOS application documentation. You must wait until you have either your employment authorization or the actual Green Card in hand. Can take months and months ..