select="document(@href)/Document/{@nodes}"/>

You seem to expect some sort of macro substitution:
 {@nodes} -> fb/section1/*
and the reevaluate the expression. XSLT/XPath doesn't work this way.

It seems you have to use Saxon's extension function for evaluating
strings as XPath expression, roughly like
 select="saxon:evaluate(concat('document(@href)/Document/',@nodes))"
Note string concatenation.
Please check your Saxon docs for the exact name of the extension
function and and the namespace declaration you need.