AY 166 12.35 LED-HEL seems ok so far. HEL-ARN leg should not be a problem either with some dozen daily frequencies.
I don't think that they will cancel LED-HEL, it is a feeder route for many intercontinental flights. HEL-ARN will be ok as TTL says above.
Plan B would be to buy a train ticket to Helsinki... Around 200 miles only.
In case you do need to consider the train option, click
here for the timetables. Trains from St Petersburg leave at 7:17, 16:30 or 22:50 (night train) daily.