var rtf = File.ReadAllText(fileName);
rtf = this.RenameStyleDifferentInOnlySpaces(rtf);
var document = provider.Import(rtf);

...

private string RenameStyleDifferentInOnlySpaces(string rtf)
{
    HashSet<string> styles = new HashSet<string>();

    string pattern = @"{\\(?:\*\\c)?s([0-9]+)[^}]*\n?[^}]*\\[^' ]* ?'?([^;]*)";

    var matches = Regex.Matches(rtf, pattern);
    foreach (Match match in matches)
    {
        string styleName = match.Groups[2].Value.Replace(" ", string.Empty);
        if (styles.Contains(styleName))
        {
            styleName = this.ReplaceOldStyleName(ref rtf, styles, match).Replace(" ", string.Empty);
        }

        styles.Add(styleName);
    }

    return rtf;
}

private string ReplaceOldStyleName(ref string rtf, HashSet<string> styles, Match match)
{
    string oldStyleName = match.Groups[2].Value;
    StringBuilder styleNameBuilder = new StringBuilder(oldStyleName + "0");
    while (styles.Contains(styleNameBuilder.ToString().Replace(" ", string.Empty)))
    {
        styleNameBuilder.Append("0");
    }

    string oldMatch = match.Groups[0].Value;
    string newMatch = oldMatch.Replace(oldStyleName, styleNameBuilder.ToString());
    rtf = rtf.Replace(oldMatch, newMatch);
    return styleNameBuilder.ToString();
}

RtfFormatProvider provider = new RtfFormatProvider();
var rtf = File.ReadAllText(ofd.FileName);
rtf = this.ReplaceSelfBasedOnStyle(rtf);
this.flowDocument = provider.Import(rtf);

...

private string ReplaceSelfBasedOnStyle(string rtf)
{
    string pattern = @"{[\n]*\\s[0-9]+[^;]* \\\w* (Normal);}";

    var matches = Regex.Matches(rtf, pattern);
    foreach (Match match in matches)
    {
        string oldValue = match.Value;
        string newValue = oldValue.Replace(@"\sbasedon0 ", string.Empty);

        rtf = rtf.Replace(oldValue, newValue);
    }

    return rtf;
}

System.InvalidOperationException: 'Start and end inlines should belong to paragraph in one block container.'

        static void Main(string[] args)
        {
            DocxFormatProvider _DocXProvider = new DocxFormatProvider();
            string outputFile = $@"..\..\MergedResult.docx";
            File.Delete(outputFile); 

            RadFlowDocument target = _DocXProvider.Import(File.ReadAllBytes(@"..\..\HeaderFooter.docx"));
            RadFlowDocument source = _DocXProvider.Import(File.ReadAllBytes(@"..\..\SubHeaderFooter.docx"));
            RadFlowDocument contentSource = _DocXProvider.Import(File.ReadAllBytes(@"..\..\ContentOnly.docx"));

            MergeHeaders(target, source);
            MergeFooters(target, source);


            MergeContent(target, source);
            MergeContent(target, contentSource);
            System.IO.File.WriteAllBytes(outputFile, _DocXProvider.Export(target));
            Process.Start(outputFile);
        }

        private static void MergeContent(RadFlowDocument target, RadFlowDocument source)
        {
            DocumentElementImporter importer = new DocumentElementImporter(target, source, ConflictingStylesResolutionMode.UseTargetStyle);

            foreach (Telerik.Windows.Documents.Flow.Model.Section section in source.Sections)
            {
                foreach (BlockBase block in section.Blocks)
                {
                    BlockBase importBlock = importer.Import(block);
                    target.Sections.Last().Blocks.Add(importBlock);
                }
            }
        }

        private static void MergeHeaders(RadFlowDocument target, RadFlowDocument source)
        {
            Header targetHeader = target.Sections.First().Headers.Default;
            Header sourceHeader = source.Sections.First().Headers.Default;

            DocumentElementImporter importer = new DocumentElementImporter(target, source, ConflictingStylesResolutionMode.UseTargetStyle);
            foreach (BlockBase block in sourceHeader.Blocks)
            {
                BlockBase importedBlock = importer.Import(block);
                targetHeader.Blocks.Add(importedBlock);
            }
        }

        private static void MergeFooters(RadFlowDocument target, RadFlowDocument source)
        {
            Footer targetFooter = target.Sections.First().Footers.Default;
            Footer sourceFooter = source.Sections.First().Footers.Default;

            DocumentElementImporter importer = new DocumentElementImporter(target, source, ConflictingStylesResolutionMode.UseTargetStyle);
            int i = 0;
            foreach (BlockBase block in sourceFooter.Blocks)
            {
                BlockBase importedBlock = importer.Import(block);
                targetFooter.Blocks.Insert(i++, importedBlock);
            }
        }

string rtf = File.ReadAllText("inputFile.rtf");
rtf = FixSelfReferringStyles(rtf);

Telerik.Windows.Documents.Flow.FormatProviders.Rtf.RtfFormatProvider provider = new Telerik.Windows.Documents.Flow.FormatProviders.Rtf.RtfFormatProvider(); 

 var document = provider.Import(rtf);

...

private static string FixSelfReferringStyles(string rtf)
{
    string regexString = @"{\\s([0-9]+)[^}]*\\slink([0-9]+)";

    var matches = Regex.Matches(rtf, regexString);
    foreach (Match match in matches)
    {
        if (match.Groups[1].Value == match.Groups[2].Value)
        {
            var oldValue = match.Groups[0].Value;
            var newValue = oldValue.Replace(@" \slink" + match.Groups[1].Value, string.Empty);
            rtf = rtf.Replace(oldValue, newValue);
        }
    }

    return rtf;
}

var fieldInfo = editor.InsertField("TOC");
fieldInfo.UpdateField();
fieldInfo.IsDirty = true;

document.Lists.Remove(1); 

<a></a>
<p>First paragraph</p>
<p>Second paragraph</p>
<p>Third paragraph</p>