Purpose : take docx put text from some outer datasorce to content controls (dictionary  CCtag, value to keep it simple). And remove CC from document keeping all text and formating.

Spent 3 day already. The only result I achieved - I can find and delete content controls from docx. But any try to add text lead to document corruption. Tried to insert <w:t>bla-bla</w:t>
Probably cause of error is these attributes
w:rsidR="006D4366" w:rsidRPr="00E71775" w:rsidRDefault="006D4366" w:rsidP="006D4366" which are usually present in tags. Have no idea how to generate them.

Besides the aproach with replacing CC with text is not the best one. In this case all  formatting is lost. Will be very thankful for any help.

found such solution

 PackagePart documentPart = null;
            const string documentRelationshipType =
                "http://schemas.openxmlformats.org/officeDocument/2006/relationships/officeDocument";
            foreach (PackageRelationship relationship in pckg.GetRelationshipsByType(documentRelationshipType))
            {
                documentPart =
                    pckg.GetPart(PackUriHelper.ResolvePartUri(new Uri("/", UriKind.Relative), relationship.TargetUri));
                break;
            }

            XmlDocument doc = new XmlDocument();
            doc.Load(documentPart.GetStream());

            XmlNamespaceManager nsmgr = new XmlNamespaceManager(doc.NameTable);
            nsmgr.AddNamespace("w", "http://schemas.openxmlformats.org/wordprocessingml/2006/main");

            foreach (XmlNode sdtNode in doc.SelectNodes(@"./w:document/w:body//w:sdt", nsmgr))
            {
                XmlNode propNode = sdtNode.SelectSingleNode(@"./w:sdtPr/w:alias", nsmgr);
                if (propNode != null)
                {
                    foreach (XmlAttribute attrNode in propNode.Attributes)
                        Console.WriteLine(attrNode.Value);
                    XmlNode node = sdtNode.SelectSingleNode("//w:sdtContent/w:p", nsmgr);
                    if (node == null)
                        node = sdtNode.SelectSingleNode("//w:sdtContent/w:r", nsmgr);
                    XmlNode clone = node.Clone();
                    XmlNode wt = clone.SelectSingleNode("//w:t", nsmgr);
                    wt.InnerText = "bla-bla";
                    sdtNode.ParentNode.ReplaceChild(clone, sdtNode);
                }
            }
            documentPart.GetStream().Seek(0, SeekOrigin.Begin); //reset the stream-pointer
            documentPart.GetStream().SetLength(0);
            doc.Save(documentPart.GetStream());