you should look at XSLT, it can help transform xml as you need

ClassMethod SplitXML() 
{

    for item="market","product" {
        set params("path")="/doc/"_item
        
        Set tSource=##class(%Dictionary.CompiledXData).%OpenId(..%ClassName(1)_"||XML").Data
        
        Set tXSL=##class(%Dictionary.CompiledXData).%OpenId(..%ClassName(1)_"||XSL").Data

        // Transform the source according to the XSL
        Set tSC=##class(%XML.XSLT.Transformer).TransformStream(tSource,tXSL,.tOutput,,.params)
        If $$$ISERR(tSC) Quit
        
        write !!
        // Output the result to the screen
        Set tSC=tOutput.OutputToDevice()
    }
}

XData XML
{
<?xml version="1.0"?>
<doc>
<header></header>
<product><test>1</test></product>
<market><test2>2</test2></market>
</doc>
}

XData XSL
{
<?xml version="1.0"?>

<xsl:stylesheet xmlns:xsl="http://www.w3.org/1999/XSL/Transform" version="1.0">

<xsl:param name="path"></xsl:param>

<xsl:output method="xml" indent="no"/>
 
<xsl:template match="/">
    <xsl:copy-of select="$path"/>
</xsl:template>
    
</xsl:stylesheet>
}

will output like below

USER>d ##class(Test).SplitXML()


<?xml version="1.0" encoding="UTF-8"?><market><test2>2</test2></market>

<?xml version="1.0" encoding="UTF-8"?><product><test>1</test></product>

you can do it with ccontrol tool

Syntax:
    ccontrol create <instance name> <parameters>
Description:
    Create an instance's entry in the Cache registry.
Parameters:
    directory=<installation directory name>
    versionid=<version identifier>
Example:
    ccontrol create mystuff directory=/usr/cachesys versionid=2008.2.0.357.0

In Caché there is no any such classification for errors. Developer should decide it depends on his own tasks and needs.

this port is from internal Apache which was installed by default with Caché. And should be used only in development.

For production, or pre-production, you should use some external WebServer, IIS (if you use Windows), Apache or Nginx. You can find more information about configuration all of this webservers, in the documentation.

If you would look carefully to your code, you would see this place

all HTML should be between &html< and >

But in your code, some of HTML outside of this command.

Looks like you forgot to define web application for your REST Service.  Please look at the documentation. And at this part:

Each subclass that has its own entry point must have its own CSP web application. You define the CSP web application and specify its security for on the Management Portal’s [Home] > [Security Management] > [Web Application] page, which you can access by clicking System Administration, Security, Applications, and Web Applications. When you define the CSP web application, you set the Dispatch Class to the name of the custom subclass of %CSP.REST and specify the first part of the URL for the REST call as the name of the application.

Since 2011.1 version was added %vid to achieve some paging functionality. Some more details.

SELECT *, %vid FROM (SELECT ....) v WHERE %vid BETWEEN 5 AND 10

Looks like, you call this service in the wrong way. I would suggest, that you call it by URL http://localhost:57772/csp/samples/REST.DocServer.cls while you should use this URL http://localhost:57772/csp/samples/docserver/namespaces But before you should activate application /csp/samples/docserver, for details look at the documentation.