regional accents were strong among simple people, educated people and higher classes strived to speak standard Polish
I agree with most what you said, with some exceptions: educated people from the Borderlands (Wilno, Lwów, etc) spoke with strong "singing" accent, dark L, voice H, etc.
Was there a difference in classes before the Second World War (and if so, was there any difference in accent between the classes?)
Of course, classes existed. There were peasants - both poor and wealthy. Impoverished petty gentry, often speaking different dialect and accent than the local peasants even though their lifestyle was not that different. Landowners, speaking "the received Polish". Industry workers, often former peasants, who spoke their own village dialects.
Most dialects had their characteristic accents, the way how some vowels were spoken, the way how some clusters of consonants were pronounced, the way where the word and sentence stress was put. A typical example of it is a so-called mazurzenie. So you cannot talk about accents without talking about dialects, because they are strongly connected. Here is a nice website about Polish dialects, unfortunately in Polish only, gwarypolskie.uw.edu.pl
But you can at least look at some pictures there and listen to various examples of regional dialects.
Educated Jews spoke good Polish, small town Jews spoke strange Polish (if any at all), using wrong declensions, and wrong syntax.
In this fragment of the old movie "Włóczęgi" (Tramps), you can hear traditional eastern accent from Lwów (now Lviv, Ukraine).