This will happen in most words where you have a cluster of R + those consonants. The R will influence and merge with following S, N, T and/or D, and for some dialects L too (although that's rare). That's why the part of Stockholm called Hornstull will in fact have all of its middle cluster of consonants retroflexed by the R into /ɳʂʈ/, if phonology is your thing.
yes! The Southern Swedish and the Danish / French / German /r/ are quite similar! Check out the different "Skånska" / Scanian dialects: https://www.youtube.com/watch?v=Wc3-AyPLa6I
The Finnish and the Standard Swedish /r/ are the same but in the Finnish variant, you do not merge /r/ with /s/ (nor any other consonant).
Your question was "Många is a verb?" so I thought you were asking if många is a verb.
The question word order is verb - subject - order, as you say, and interrogative - verb - subject - order if there is an interrogative.
But the interrogative doesn't have to be a single word. In this case, the entire phrase hur många pojkar is the interrogative, and the verb simmar comes after it.