I have communicated a lot with MarshalBraginsky by PM these last few days.
A for the Danish serfdom, while it is true that it was not serfdom in the strict sense of the word, but to Swedish peasants it looked that way. The collapse in royal power in Denmark from 1241 until 1340 (kingship was in flux before the 8 years without a king 1332-1340) allowed the nobility to expand on their already large powers. Common, clan, family and commune land was claimed by nobility, and farmers who had been semi-self-owning became curvee-owing tenant farmers who were hindered from leaving their birth farms.
From the early 14th century, the Danish nobility claimed judical rights on their estates, and the land owned by free-holding farmers droppes to about 10%.
The Swedish peasants owned 52% of the land and about 63% of the rural population was self-owning farmers. Tenant farmers could break contract pretty much whenever they wanted, break new land or take up tenancy with another landholder to get better terms, something Danish tenats seem to have been prevented to do.
1377, the Danish crown intervened, telling nobility they could not force tenants to stay nor intervene in their marriages (which indicates the nobility was doing so) and from the early 15th century, the Danish nobility held judical, economic and worldly power over their tenants, making it close to impossible for the Danish peasants to complain or act in their interests. As a political power defending their interests, the Danish peasants had been eradicated.
To the Swedish peasants, the Danish development looked like a slide into by the early 15th century, effective serfdom in everything but name, and they were deadly afraid of being subjected to the same development in Sweden. As they were armed, had rights and were a strong political force in Sweden (with their position at the things, the landsmöten and later the riksdag) they used their substantial power to retain those rights. The Danes failed to understand this, as they and their Frisian and Low German mercenaries were not accustomed to it.
This is one of the most substantial inherit flaw in the Kalmar Union, and I maintain that the easiest way of avoiding it is to stop the Danish nobility from acquiring large estates and retain the Danish peasants as a political force. That way the Union King can work with the Union peasants, regardless of where, to centralise and cement power thre way the Wasas did in Sweden OTL 1523 to 1632.