Do you think it could be borrowed from MSA rather than inherited?
No, I highly doubt it.
If not, then I hope you or anyone acquainted with dialects that have إن inherited from Arabic provide us with examples.
I don’t know how to prove it, I only have my word. Perhaps I’m wrong, but in Palestinian Arabic I’m quite sure. My grandparents almost never used إذا, at least I don’t recall them using it, it was always إن, some 40 years ago I would only hear إن from Palestinians. Today, I hear إذا quite a lot, usually pronounced إزا, although إن is still used. However, some segments of the society don’t use إذا, specifically those that speak a rural dialect (فلاحي). From the top of my head I can remember two proverbs that use إن:
إن كان صاحبك عسل ما توكولوش كله
إن كبر ابنك خاويه
The first proverb exists in Iraqi Arabic as: إذا چان صاحبك عسل لا تاكله كله. I don’t remember if there is an Iraqi version of the other one.
Having said that, I decided to look up a couple of songs in Egyptian Arabic that I remembered use إن, the songs are:
إن كنت ناسي أفكرك لهدى سلطان
إن كنت غالي عليا لعايدة الأيوبي
إن حبيتني أحبك أكتر لفريد الأطرش
إن راح منك يا عين لشادية
So it seems that I was right about Egyptian Arabic. But I also found:
إن كنت ناوي تعذبني لعبّاد الجوهر
إن رحت لا عاد تنتظر مني أجيك - من أغنية الله عليك لعبد المجيد عبد الله
So it seems that it’s also used in at least some Saudi dialects as I expected.