I've watched the WR for Jill Major Glitches and tried following it exactly, but the Plant 42 skip seems extremely difficult to replicate. Is there a sure-fire way to get the glitch? Or is it genuinely just really hard to get right?

Plant 42 skip is pretty much the hardest skip to get in the game. You have to leave the room at the same frame that Plant 42 takes fatal damage, then go back in, load your bazooka with flame rounds (You can use grenade around if you want, but you have to hit 5 times from my testing in order for it to work as opposed to the 2-3 with flame rounds) which gets you control of Jill, then move near the fireplace without being able to see where you are, and shoot the tentacles. Then you need to get the key and leave the room before the cutscene picks back up. There's a chance you can just straight up miss the vines as well.

There is a slightly different way you can go about doing it by using text buffering to glitch the cutscene. It's slightly slower but it might be a bit more consistent in getting the beginning of the glitch to work. Here's a video of the other method of getting the skip.