I don’t think the problem is with PDF documents. They should be reliably cross-platform.
Can you post a LayOut document with CMYK images? I have no idea what happens when it is opened in the Windows version.
Generally, there is no problem converting CMYK to RGB. The other direction is different, as CMYK, as you probably know, has fewer colours than RGB as it is limited to what can be achieved by dithering the four printer inks.
That is correct, the PDF itself isn’t any problem.
As far as I have found out, Fills in CMYK, created on Mac, opened on Win seems to be the same or simiar, but opening the file then again on mac, the CMYK colours are not anymore the same than initally. That turns out for me, that the files are not really interchangable…