Here are some notes to point out I picked out of Christfrid Ganander's book Mythologia Fennica (1789).
The multiple references to Fornjotr and other "Finnish kings": I think that seeing these figures as actual Finns is false. Many of these are forces of nature personified as giants, and any "Finn" referenced in them is more likely to mean Sámi.
Akka, wife of Ukko: In many places in Finland, Sämpsä is the one who sowed trees. But in some others, it is actually the mother of trees, who is called Kave or Kapo... It does really make you think like, what is going on. But I think Sämpsä's original role is the sowing of fields. He is also (probably) the son at the bottom of the field, and he had intercourse with his step-mother. This step-mother has been assumed to be Maatar, because a "wife underground" is mentioned alongside the son at the bottom of the field. After this, Ukko is asked to bring rain. Mikael Agricola wrote that for fertility, the wife of Rauni-ukko got horny and Ukko "splashed nobly from the bottom". Splasing could refer to rain, of course, but the location of "bottom" sounds more like where Sämpsä is. Well, however you put it, it seems that Ganander's assumption that Ukko's wife is Maatar isn't unreasonable at all. Runosongs just don't support that she was horny for Ukko, but for her step-son Sämpsä.
Aluenjärvi: Other names Alamanjärvi, Alvajärvi, Liemonvesi. Lakes of the name Alamanjärvi also have existed in Pudasjärvi, Pedersöre and Ilmajoki (all in Ostrobothnia); with the name Alvajärvi in Viitasaari (Central Finland).