(1-2) SLAMM skips the cells that are blank. In other words elevations are not updated, inundation and wetland conversion are not executed. So basically nothing happens for those blank cells while if they were initially dry land they could convert to wetlands as sea level rises.
(3) In the latest version you have, SLAMM 6.2, there should be a conversion button, "blank" to "dry land", in the Edit Cell menu. This will fill ALL blank cells as undeveloped dry land unless you have also an impervious raster in your project. If this is the case, then some cells may be converted to developed dry land if the pct. impervious is greater than 25. Such button only works if you are tracking ALL the cells in your project.
Hope this helps.