Sorry to rain on your parade but:
- William was not very popular in Poland IOTL which was one of the reasons he never married Jadwiga; Jawiga herself was 13 in 1386, so her opinion did not count, although it is true that she wanted to marry William; but OK, let's say that Władysław Opolczyk managed to make this marriage happen (he was actually quite close to it).
- In 1406 the ruler of Scandinavia was de facto Margaret I, who chose the wife for Eric in 1406 - Philippa of England; Margaret would have needed some reason to change it, but a chance for Polish crown might be tempting enough.
- I have serious doubts about Eric being capable to keep 4 crowns at the same time - he was unable to keep 3 crowns IOTL (after Margaret's death), and that should have been easier for him; he was allegedly harsh, unpleasant man, who kept offending his subjects. But hey, Jadwiga was a strong personality too and she might be a good influence on her husband. Or not. I have my doubts
However, and alliance/union between Poland and Kalmar Union might have had some interesting consequences
- Poland most probably returns to Baltic Sea, at least indirectly; the dukes of Pomerania were Eric's relatives and becoming intermediaries for Polish trade would have been an attractive offer for them.
- Poland supports Eric against Holstein, which might help him win, which boosts his prestige and reduces costs of the conlict suffered by his Scandinavian subjects IOTL
- Scandinavia and Pomeranian duchies support Poland against the Teutonic Order (if the conflict starts again) - combined forces of Scandinavia, Pomerania, Masovia and Poland are, IMHO, even more powerful than Polish-Lithuanian alliance IOTL; another interesting thing is that the Order wouldn't be able to claim the Lithuanians are pagans, since they do not participate in war; of course you need to think about consequences of Jogaila NOT marrying Jadwiga for Lithuanian internal politics; anyway, if the war happens, I would put my money on Eric.
- Polish-Teutonic conflict might not happen at all, since Poland now has access to sea via Pomerania; however, the conflict was NOT about an access to the sea, but about land taken by the Order from Poland in early XIVth century and permanent cold war between both states; Danzig and opening Vistula as a trade road to the sea was simply a cherry on the top of an old feud; therefore, I think the war with the Order will happen.