More tracks could be added in the area of the "rock cut" in any of several different ways. None of them would be as easy as just laying down the track, and would probably engender some environmental objections due to filling in some of the Harlem river canal or whatever the official name of that waterway is. There is plenty of waterway to give up 10 or 15 feet of, rather little river traffic, particularly of any significant size, and probably some could come from the landward side anyway, by shaving down the slope or undercutting a semi-tunnel into the cliff.
West of Van Cortlandt Park is more like the territory that would patronize the Riverdale or SD stations; Marble Hill is south of that park.
Marble Hill to Yankee Stadium is the main area of interest and there are plenty of people living along that route who could patronize the stops along that stretch. The terrain is kind of vertical, with the railroad at the bottom more or less alongside the river and a lot of apartment buildings on the hillside, with a freeway in between. Not at all impossible for pedestrians to get to the stations, but not ideal either, especially as there are two subway lines at the top of the hill that are cheaper and offer better connections and also some bus lines. Nonetheless, MN is probably a better choice for some, at least, the exact number varies with the MN fares and the specific point in the neighborhood one is starting from / going to.
From what I've seen of these areas, Coop City is the only part of the Hell Gate station-territories with anywhere near the number of potential riders that could use the station along the Harlem, and the station is at the tip of Coop City compared to the RR; the rest is not particularly within walking distance, so you are already talking about a 2 seat ride and you are going to Penn Station, probably in need at least one more seat.
I don't really think that either of these sections are going to provide a lot of passengers, unless the fares are dropped to subway levels - and maybe not even then - and even if then there would also be other difficulties if the idea were too successful.