I don't speak Russian, but the construction of the patronymic follows rules for suffixes that may involve changes in the final syllables. This happens in other languages as well.
As for the patronymics, foreign princesses would be given new patronymics when their father's names did not exist in Russian (such as
Christian August) - usually it was Feodorovna, after the Feodorovskya Icon. In
Catherine's case, her new patronymic was probably an hommage to
Catherine I (
Marfa /
Yekaterina Alekseyevna); it may also have been chosen out of devotion to St
Alexis or the many Alexis/Aleksey in the imperial family.
This message was edited 7/26/2017, 4:19 AM