Älvdalsmål is critically endangered. You have the parallel existing älvdals dialect, which most people living in Älvdalen speak. Last I checked there was something like 5 living people who can speak älvdalsmål.

I'd argue that the reason locals understand you more than you understand them is, in these cases, that they're effectively bilingual. If they want you to understand then they'll switch to Swedish and you'll understand just fine.

There hasn't been anyone speaking only Mål in a few generations, in my estimate. You either speak both mål and Swedish, or only Swedish.

And no, you don't pick these up easily. I grew up in Rättvik. My grandmother used to speak rättviksmål on occasion (she was bilingual with Swedish) I can understand rättviksmål somewhat. I used to date a girl from Malung, who spoke Swedish usually, but exclusively Malungsmål with her mom. 3 years together and I still couldn't understand a single word she said to her mother. Mål is often conflated with the dialects of the same area, but they are 2 distinct things. Skånska is a dialect,I can understand it fine, even I have to focus a bit more than usual. Dalarna has a dialect too, the one Gunde Svan speaks on TV, it's easy to understand. Mål is separate, and much, much harder.

Rättviksmål is considered the easiest to understand for native speakers of Swedish. Here's a reference text for your perusal: https://shfstor.blob.core.windows.net/rattvik/uploads/images...

This Wikipedia article is also interesting.

They're essentially separate languages springing from the same roots, and therefore have some similarities.

https://sv.wikipedia.org/wiki/Egentliga_dalm%C3%A5l

You're right that Sami is harder though. It does not share a common root with Swedish, so there are basically no similarities. Even German would be easier for Swedes as they're both Germanic languages, but they've diverged long enough ago that similarities are sparse these days.