EWS: Cannot append HTML formatted text to HTML body when using CreateItem to forward an email

Question

I want to append some text to the beginning of the body of an email I'm forwarding (via a web Mail Addin/JavaScript for Office) using CreateItem with a ForwardItem element. The text I'm inserting is complex and seems to generate various ErrorSchemaValidation errors. Here's what I've tried for the value of the NewBodyContent element (with BodyType="HTML"; the body text is passed via the headers parameter):

• Used complex text (in this case the full Internet Message headers from another email) - fails, no text is appended
• Used encodeURI on complex text - works, but all the encoded values are not transformed to HTML and the appended text appears as Plain Text (although source body is still HTML; e.g. the appended text reads as "From:%20username %20%3Cuser@domain.com%3E%0A" instead of "From: username <user@domain.com>")
• Inserted 'Hello world!' - works
• Inserted '<H1>Hello world!</H1>' - fails, no text is appended

I'm baffled. Any ideas on how to properly append complex text to the HTML body?

function forwardItemRequest(item_id, email, changeKey, headers) {
    var request;

    // The following string is a valid SOAP envelope and request for forwarding
    // a mail item. Note that we use the item_id value to specify the item we are interested in,
    // along with its ChangeKey value that we have just determined
    // We also provide the XML fragment to specify the recipient addresses

    //TEST 5: Try with HTML content
    request = '<?xml version="1.0" encoding="utf-8"?>' +
        '<soap:Envelope xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"' +
        '               xmlns:m="http://schemas.microsoft.com/exchange/services/2006/messages"' +
        '               xmlns:xsd="http://www.w3.org/2001/XMLSchema"' +
        '               xmlns:soap="http://schemas.xmlsoap.org/soap/envelope/"' +
        '               xmlns:t="http://schemas.microsoft.com/exchange/services/2006/types">' +
        '  <soap:Header>' +
        '    <RequestServerVersion Version="Exchange2013" xmlns="http://schemas.microsoft.com/exchange/services/2006/types" soap:mustUnderstand="0" />' +
        '  </soap:Header>' +
        '  <soap:Body>' +
        '    <m:CreateItem MessageDisposition="SendAndSaveCopy">' +
        '      <m:Items>' +
        '        <t:ForwardItem>' +
        '          <t:ToRecipients>' + email + '</t:ToRecipients>' +
        '          <t:ReferenceItemId Id="' + item_id + '" ChangeKey="' + changeKey + '" />' +
        '          <t:NewBodyContent BodyType="HTML">' + headers + '</t:NewBodyContent>' +
        '        </t:ForwardItem>' +
        '      </m:Items>' +
        '    </m:CreateItem>' +
        '  </soap:Body>' +
        '</soap:Envelope>';


    return request;
}