Nom, what are these accounts of mixed swimming? The steipler wrote of American jews that he was shocked to his core that people who are shomer shabbos can even think of doing such a thing, which he says is yehereg velo yaavor. It was clearly a big deal and something he never heard of, and he was from Europe. It’s in kiryana de’igrasa. I don’t know what naskilish books you read that led you to think otherwise, but if you know of a rov who wrote nonchalantly about mixed swimming, please do tell. I’m sure there weren’t any.
I don’t know how you can group together individual spiritual accessories, such as uman, with mitzvos deoraysoh like peter chamur – there are jews who own donkeys; in Europe, that wasn’t common… simple as that.
And yes, MO was born of haskalah. Its leaders were all soaked in it, including revel and belkin. Their attitudes are taken straight from maskilim who believed in the independent importancd of secular studies and culture. The kind of stuff rav hirsch spent decades fighting.