1) Works perfect:
"([क-ह]|[क़-य़])ρ","ρ\1" 
"((([क-ह]|[क़-य़])्)+)ρ","ρ\1"
"i((([क-ह]|[क़-य़])्)+)","\1i"

2) Works perfect:
"i([क-ह]|[क़-य़])","\1ि" 
"([१३])(([॒॑])+)","\1"

3) Does not work at all:
We look for "([॒॑])([ा-ौ]|[ॢॣ]|[ँंः])" and replace it with "\2\1"
"(([ा-ौ]|[ॢॣ]|[ँंः]|[॒॑])+)ρ","ρ\1"

(()())

Word.Find find = app.Selection.Find;
                    string finds = listView1.Items[s].Text; // строка для поиска
                    find.Text = finds;
                    string fonts = txB_NAME_FONT.Text;
                    find.Font.Name = fonts; // поисковый шрифт
                    find.Replacement.ClearFormatting();
                    string Repl = listView1.Items[s].SubItems[1].Text; //строка для замены
                    Repl = Repl.Replace("\r", string.Empty); 
                    find.Replacement.Text = Repl;
                    Object wrap = Word.WdFindWrap.wdFindContinue;
                    Object replace = Word.WdReplace.wdReplaceAll;
                    if ((s == 14) || (s == 199))
                    {
                        find.Text = "";
                        if ((finds.IndexOf("[") > 0)||(finds.IndexOf("]") > 0))
                        {
                            find.Text = "";
                            find.Text = finds;
                        }
                        else
                        {
                            find.Text = "";
                            find.Text = "[" + finds + "]"; //оборачиваем в скобки, иначе исключение для регулярок
                        }
                        find.Execute(FindText: Type.Missing,
                         MatchCase: false,
                         MatchWholeWord: false,
                         MatchWildcards: true, // включаем регулярки
                         MatchSoundsLike: missing,
                         MatchAllWordForms: false,
                         Forward: true,
                         Wrap: wrap,
                         Format: true,
                         ReplaceWith: missing, Replace: replace);
                    }