I am sorry to hear about your trouble with your paper. Unfortunately, from experience, it is not that unusual to wait 14 months or longer for a referee's report.
It is certainly fine to email the editor politely, as Karl says, and inquire about the status. You could also ask whether the journal has had any preliminary comments on the article from the referee. (I.e., the referee may be willing to state that they think the result is interesting, and they are likely to recommend acceptance if no issues with the paper come to light. Indeed, many journals now do solicit such comments from referees.)
In my view, the editor should be a bit embarrassed about not responding to your request in September. However, if they are doing their job, they are already hassling the referee and trying to get them to agree to a date by which the report will be provided.
If you do not hear back from the editor in a reasonable timeframe after your e-mail, you might want to e-mail the main journal e-mail address, mentioning that you have tried to contact the editor but have not been successful, and asking whether they know something about the progress of the paper. That way, you will make sure that the journal's secretary knows about the issue, and will follow it up. (However, do make sure to be polite and not to sound critical of the editor, which would be counterproductive.)
Finally, you are absolutely right that it would be tough to get a rejection letter after this time - although it is certainly not unheard of. You would hope that, if the referee was going to reject the paper, they would have done so quickly. However, if the report is negative, there is little that the journal can do.
If the report does recommend acceptance, then I would expect that the journal would be inclined to follow the referee's advice.