Why did KiCad merge my pins into one open blob in the solder mask gerber?
You will need to look at your soldermask minimum width setting.  The soldermask is not yet WYSIWYG in pcbnew (see https://bugs.launchpad.net/kicad/+bug/1812096 and https://bugs.launchpad.net/kicad/+bug/1784027)

In the meantime, go into pcbnew->Setup->Pads and mask clearance to set the correct values.  If you want the pads to be separated, you'll need to specify a value that is less than the pitch.