Norm, it may not be possible to find a good answer for your question. I don't know the history of name laws in Finland after the separation from Sweden, but in Sweden there wasn't a true name law before 1901. The 19th century (particularly the second half) was a time when the patronymic naming system was being abandoned, as families or indviduals opted for choosing a "surname". This process was not organized, it was just a cultural trend, fueled by the increasing geographic and social mobility of the time.
If you don't have any evidence of the name Liljeqvist being used in Helena Fredrica's family earlier, she may very well have adopted it recently. We can only guess at the reasons.
