In the sentence, "I saw him coming here",

(1) Must a verb (regardless finite or non-finite) ALWAYS have a SUBJECT?
(2) Does "coming" have a subject? If yes, which is the subject?

(3) Can an OBJECT of a verb be simultaneously the SUBJECT of another verb? "Him" is the object of "saw"; is "him" simultaneously the subject of "coming"?

In the sentence, "I saw John who was coming here" or "I saw him who was coming here",

(4) "John" or "him" is the object of "saw" but not the subject of "coming", because "who" is the subject of "coming". This merely avoids Questions (1), (2) and (3) above.