This should get you started.  It's not completely as you asked, but I ran out of time.  If you've never used windowing functions then you'll need to read about them and understand how they work.

WITH NumberedforCompletion AS (
    -- Get the no of completions so far per Identifier
    SELECT *
    , SUM(CASE WHEN SN_Status = 'COMP' THEN 1 ELSE 0 END) OVER (PARTITION BY Identifier ORDER BY CompletedDate ROWS BETWEEN UNBOUNDED PRECEDING AND 1 PRECEDING) AS CompsSoFar
    FROM #temp
    where SN_Type = 'Re-Activattion'
    ) --select * from NumberedforCompletion
, ReverseNumbered AS (
    -- Reverse the numbering so that the most recent is numbered 1
    SELECT *, DENSE_RANK() OVER (PARTITION BY Identifier ORDER BY COALESCE(CompssoFar,0) DESC) AS ReverseNo
    FROM NumberedforCompletion
    )
, DateExtremes AS (
    select distinct Identifier,
    MAX(CompletedDate) OVER (PARTITION BY Identifier, ReverseNo) AS FinishDate,
    MIN(CreatedDate) OVER (PARTITION BY Identifier, ReverseNo) AS StartDate,
    SUM(CASE WHEN SN_Status = 'SENT' THEN 1 ELSE 0 END) OVER (PARTITION BY Identifier, ReverseNo) AS SentCount,
    SUM(CASE WHEN SN_Status = 'N-CO' THEN 1 ELSE 0 END) OVER (PARTITION BY Identifier, ReverseNo) AS NCoCount
    from ReverseNumbered
    )
SELECT
    Identifier
,    StartDate
,    CASE WHEN StartDate > FinishDate
        THEN NULL
        ELSE DATEDIFF(day,StartDate,FinishDate)
    END AS RE_ACT_COMP_TIME
,    NCoCount
,    SentCount
FROM DateExtremes

John