ok, I downloaded sqplus and checked.
SetValue() uses sq_rawset() that internally does a newslot
the get fails because SquirrelVM::CreateFunction() seems to be buggy... at least the overload that you are calling. It creates _get & _set with a wrong typemask/paramcount(so _get fails). I'm not really sure what that function is trying to do and why sneaks in some additional typemask stuff.
Anyway, other than squirrel does what is supposed to do.
ciao
Alberto