I am a linguist.
as for the 1st question, regarding the “ł”, it’s a result of a sound change which occured between 14th and 16th century in the Slavic world.
You are already familiar with the fact that everything evolves – languages evolve towards simplicity; towards the “economy of speech”; and that is how the sound changes appear – the speakers simplify their speech.
This particular change – labeled as a “change from L to O/U” occured in multiple Slavic languages; and it’s easily observable:
For example, the Old Slavonic word for “long”: “длъгъ” is “долгий” in Russian; “dolgi” in Slovene; “dlouhý” in Czech, and “dlhý” in Slovak (there we see the first group of languages, which hasn’t gone through this change)
However, in Polish, the word is: “długi”; in Sorbian “dłujki/dołhi” in Belorussian “доўгі”, in Ukrainian “довгий” (this is the second group)
And, finally, in Serbian: “дуги” and Croatian: “dugi” (the languages which undergone the change).
So that’s how this change has gone: L -> Ł -> U
The L got more and more soft, and eventually turned into U.