Frost and Farabee don't even play the same position. Frost is the one getting the PP time right now, but that hardly qualifies as "passing him on the depth chart," especially as Frost plays more sheltered minutes and gets less 5-on-5 time than Farabee despite centering the #1 line.
Farabee's overall TOI/G has gone down since Frost was called up 8 games ago due to the PP usage, but his EV TOI/G has actually gone up by 21 seconds per game. In those 8 games, Farabee's EV TOI has ranked 3rd, T-9th, 8th, 9th, 6th, 5th, 5th, 6th of the forwards - so even at its worst, still not 4th line minutes.
Sure, 4th line with no special teams wouldn't be ideal for a 19 year old. But it's not happening right now and despite your hypotheticals - JVR "find[ing] his way back up the lineup" or "Patrick return[ing]" - it's also possible that Farabee reclaims his PP spot, or Frost gets sent back to the AHL, or one of their top 6 guys gets injured, or the guy with a horrible concussion history and no return date doesn't return soon or isn't up to par when he does, etc. The point being is a lot of things could happen and not all of them are bad for Farabee.