Joining 4 tables of which one is unrelated
Problem 1:  LIMIT is somewhat meaningless without ORDER BY to indicate which 25.

Problem 2:  Don't say x=null, it won't do what you think.  Instead do x IS NULL.

Problem 3:  Do you really want a "cross join"?  That is, a JOIN without an ON clause, and hence gets all combinations of the two tables?

Problem 4:  Well, I did not check the rest of what you have.

Back to your question...  The pattern is

( SELECT ... ORDER BY ... LIMIT 25 )
UNION ALL  -- or DISTINCT, depending on your understanding of the data
( SELECT ... ORDER BY ... LIMIT 25 )
ORDER BY ... -- Yes, again
LIMIT 25;


If you need an offset of, say 100, then:

( ... LIMIT 125 )
UNION ...
( ... LIMIT 125 )
ORDER BY ...
LIMIT 100, 25;