The "%Inline" part tells me that it can have child nodes from the "% Inline;" entities element list.
The span element is included in the list of elements classed as "% Inline", that combined with span allowing "%Inline" child elements tells me that span is a valid child of span.
Yes it will. You can help yourself by using the w3's validator direct input option:
Absolutely.
Here's the definition from an XHTML-strict DOCTYPE for a span element.
The "%Inline" part tells me that it can have child nodes from the "% Inline;" entities element list.
The span element is included in the list of elements classed as "% Inline", that combined with span allowing "%Inline" child elements tells me that span is a valid child of span.
SPAN can contain only inline elements, such as SPAN etc.