XML schema validation and namespaces

Hi All

Correct me if I am wrong anywhere in the following
One can comopse an XML schema using multiple schemas and multiple namespaces
so that definitions can be reused (makes perfect sense). 

For this purpose XML schema can have references to other schemas which might
have different namespaces. This is done by 'import' / 'include' tags in the
schema definition. Q1: what are the possible ways for doing the same.  

The instance document reffering to this schema would be validated by loading
other schemas and looking for corresponding definitions in those schemas.
(good so far)

Now XML instance document can also contain references to multiple namespaces
and schema using schemaLocation and
noNamespaceSchemaLocation ( why multiple ??) 

Q2: XML document ( which of course must have a root element) would have its
definition in one schema only. the child elements which in case refer to
other schemas must be refered in the schema only.
Second for a validating parser what would be the precise set of rules for
locating the definition of an element that is mentioned in the instance
document.

Now to make things worse each element can refer to a namepace and supply the
prefix there itself (No idea where it is going now !!)  Q3: now how would
the validating parser locate the definition of this element. and Q4: why
would someone put a namespace with an element in the instance document and
not in its schema ?

looks like some of the constructs have been designed with non-validating
parsers in mind and others for validating ones. could someone clear things a
bit here.

thanks and regards
Sanjay