You call "sitting in front of you" a phrase, and "seeing an accident ahead" a clause. This anomaly explains how you get a phrase from a clause.
"2. The boy sitting in front of you is my cousin." Here "sitting in front of you" is participial construction acting adjectivally. That's how you make a participial X equate with an adjectival X.
It's well-known that a participle can act as an adjective.
I can't find anything noteworthy about your argument.
English confuse meGuest