(I started looking into this inyan a little) For what it is worth, the gemara in moed katan, yud ches amud beis, brings down the same thing about the bas kol, but instead of 40 days before yetiras havlad, it says “bchal yom vyom”. Not sure why the different girsa or if any meforshim explain the difference.
Also, there is a (famous?) Teshuvas Harambam (siman tav lamed vav)to “Ovadia the Ger”, he discusses shidduchim as it relates to a persons bechira (even though there is a bas kol) as part of the broader discussion of hakol bidei shamayim chutz miyiras shamayim. I wont venture to write how the Rambam explains this without thoroughly going through it.