Hi, I hope these notes are helpful:
RE; #1, that "catchy phrase" is called a "hook" and it's considered essential to pop music and some Jazz. Pop music is not really expected to heard by an audiophile studiously listening to an exalted system, but expected to be played in noisy, distracting places. Places like cars, businesses, restaurants, bars, and places where people may be distracted. "Milking the Hook," or repeatedly employing it in an arrangement, is a method of trying to break into a listener's attention so that they may add it to their streaming playlist or possibly buy a copy.
RE: #2 This would refer to a musical hook, which might be similar to variations on a theme in classical music, except in pop music it probably won't vary.
RE: #3 I have nothing to offer for this one.
RE: #4 Just a poetic style that in pop music helps capture an audience.
Not trying to be snarky and i hope this helps.