Blankan - Polska från Sörmland 

yes, so characteristic

Not one of the two guys playing, but stumbled over your comment: You're right, "från" means "from". Polskas often have no name (especially the traditional ones) and their most often called by the place they're from (från), who it is in tradition after (efter, for example Polska efter Byss-Calle) or who wrote the tune (av, for example Polska av XYZ). Can't recommend a book to you... best is to listen to as much tunes as you can (RU-vid obviously is a great resource) and www.folkwiki.se is also nice.

As MrFair wrote. And there are lots of tunes out on the Webb, where you can find. There are also some music collections from different districts in Sweden, where lots of tunes has been written down. The collection is called "Svenska låtar". Different areas also have associations, like this with some tunes. halsinglandsspelmansforbund.se/hsfs-latsamling/

Here are another band where there are some good tunes. ru-vid.com/video/%D0%B2%D0%B8%D0%B4%D0%B5%D0%BE-w3tiVg5Fb78.html

It looks very similar to a tin whistle, but I am no expert...

This is a low whistle, the "bass" version of the tin whistle.

Not at all. Piece is in A major and He used low E major whistle.

Tillverkaren är Goldie. Bästa whistlarna jag spelat på /Kim

I guess that Jesus invited them in. Mabye becouse of that it sounds so cool ;)

To Ja I guess He invites everyone in.

The Catholic Church has no medieval churches in Sweden. The Jesuits have nothing to do with Solna Kyrka, unfortunately.