Let the line feed in place and enclose the cell content by double quotes. This should be accepted by Excel when rows are terminated by carriage return - line feed pairs. So your CSV file should look like:
Row1:Col1, "Row1:Col2\n with new line"\r\n
Row2:Col1, "Row2:Col2, with comma"\r\n
Row3:Col1, "Row3:Col2 with "" esacped quote"\r\n
Row4:Col1, " Row4:Col2 begins and/or ends with space "\r\n
As an alternative you may write HTML files using tables. These can be also imported by Excel.