Okay, too bad there is no marriage record document.
That said: you should lean into the argument that you would NOT have been permitted to marry if you were not single/divorced, and the simple fact that you have never been married. See what legal support, either a letter from a lawyer there or articles in press, that support this point.
I don't know what you mean by statutory declaration. It's really just a letter sworn before someone - i.e., same as an affidavit. You can do this, but because it's from you, the only interested party (except your spouse, I guess), it's ... in most respects, it's the same as a letter from you, signed by you. But if it makes you feel better, sure. (It at least won't hurt)
What about your ex-fiancé? Any records that he was never married / a statement from him that you two were never married? (I wouldn't go crazy if you can't get this, it's only a thought). Come to think of it though - the profile you created, did it list your boyfriend then's name at all? If he was not named in that mock profile, don't bother with this.
Again: your main argument is that the ONLY source anywhere stating you two were married was a mock profile, never submitted. (A profile that might not even have his name!) It's NOT unusual to create a profile to check things / score when one is not sure - and that is NOT a statutory declaration of any sort if it was not submitted. And, you cannot have been married and not divorced since you were married in a country that doesn't permit that/has a registry. Etc.
If this seems too much for you: get a consultation with a lawyer or have a lawyer write the letter. It will cost money but may be worth it (at least for your mental health and stress). A lawyer might put in some more meaty language about burden of proof and lack of evidence beyond the (flimsy!) claim that a mock profile indicates something; I wouldn't try to make a legal sounding argument on my own.