hum. On Windows at least, just drop-in replace the sqlite3.dll per the new sqlite-3.25 dll and the window functions just works.
==> I don't see the need for your latter point, at least on Windows.

checked with sqlite_bro on Python-3.6.7rc1 with example:
CREATE TABLE t2(a, b);
INSERT INTO t2 VALUES('a', 'one'), 
                     ('a', 'two'), 
                     ('a', 'three'), 
                     ('b', 'four'), 
                     ('c', 'five'), 
                     ('c', 'six');
SELECT a                        AS a,
       row_number() OVER win    AS row_number,
       rank() OVER win          AS rank,
       dense_rank() OVER win    AS dense_rank,
       percent_rank() OVER win  AS percent_rank,
       cume_dist() OVER win     AS cume_dist
FROM t2
WINDOW win AS (ORDER BY a);