Why would you need an alt to participate in the valentine's day event? Of course that they mods might be able to detect who's alt is that and they will ban you from the contest. There's no rule against having an alt (if you're not banned of course) but using it for an event is imo bannable.
Typically the users who ask questions such as these, have already done it... And yes, you would be disqualified from the contest upon discovery. Question kind of answers itself.
You'd have to have put in a ton of effort already in order to get away with something like that (i.e. long-term alt that has numerous posts and a join date from some time ago; plus you would have to make it look like it's own user by only using it on an IP that your main never uses; and you'd have to be one desperately bored and lonely person to do all of that just to have your own Valentine and win a contest)
Obviously. Just like you can't make an alt to spam with yourself, or you couldn't enter the lottery with more accounts, or you can't vote for your own entry in contests etc.