This has been one of my most troublesome mobility exercises in GST (HS/PE2>iM). Been working on these for over 1.5 years now, close to 2 years.
Since I've been working on them for such a long time, I want to share some tips and suggestions on what worked for me and what didn't. Majority of these tips are stuff that I personally used from trial and error (but draw inspiration from various GBrelated sources.) These are what worked for me, but these are from a sample size of 1 person. 
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Order of progression:
I feel it's worth repeating that you start with a grip you are comfortable with, then slowly work on bringing your grip to shouder width apart, before starting over with a harder weight and a wider grip. However, I personally felt like if this was the only thing I did, it would have taken me much much longer to master this exercise. Of course, for the exercises I did to master Shoulder Extensions, I still follow the idea of moving your hands closer together before increasing weight. (Also worth mentioning that if you can't get the bar to shoulder height, chances are your grip is too narrow).
Why it did not work for me:
I personally felt like the standard approach of "only" doing shoulder extensions by reducing grip width is that I find it very hard to get into the correct position (which is bringing the bar shoulder height). The exercises I used allowed me to get into the correct position earlier.
So is it a strength issue or a flexibility issue?:
In my opinion, it can be both. There have been instances where I was doing shoulder extensions where I get a cramp in the medial head of the triceps. There's also been other instances where I get DOMS the next day on the medial head. Curiously enough, there's a specific variation of Body Levers in the Foundation courses that gives me DOMS in the exact same area. Usually, I find that it's a strength issue if I have difficulty getting into the correct position, but it doesn't really hurt.
What to do if it's a flexibility issue?:
If it's a flexibility issue, I found that I usually have some sort of unnatural discomfort either in the front of the shoulders or the elbows itself. The areas that I try to target are the pecs, anterior delts, and the biceps. I'll leave the specific discussions of how to maximize the stretch for either Foundation/GB online classes/seminars, but I personally found that (not sure how to explain the stretches better without going into content outside H1 forum):
If you're doing the stretch face down on the floor with with elbows fully locked, palms facing down, targets the biceps the most.
If you're doing the stretch face down on the floor with shoulder/elbow 90/90 in external rotation (so palms on floor) targets the pecs the most.
If you're doing the stretch face down on the floor with shoulder/elbow 90/90 in internal rotation (so palms away from floor) targets the anterior deltoid the most.
These 3 stretches can all be found in either Foundation or in online classes.
How do I get the bar to shoulder height by myself?
This is an exercise I got from a GB coach. It's the same as in this video:
However, instead of coming all the way back down to standing, pause when the bar is shoulder height. Reduce grip over time.
Is it worth using a partner to fast-track progress?
From talking to multiple GB coaches, in general it's frowned upon to have a spotter at home because of the increased risks of injury from an untrained spotter, and spotting incorrectly can slow your progress. However, personally for me, it helped my progress. I also just wanted to again bring up the part about feeling any discomfort in elbows or shoulder, that usually means your grip is too narrow.
Progressions:
1. Have the partner assist you by slightly holding the bar for the entire duration
2. Have the partner get you into the position first by sliding your fingers, then once you reached your limits, have the partner let go of the bar, and do your best to prevent the bar from dropping.
3. Without assistance of a partner, get the bar into position exactly as shown in the HS/PE2>iM video.
Is it possible to cheat on this exercise?
What I've noticed is that my thoracic spine gets very flexed and my neck protracts out very far. I'm not sure if this is a result from doing the progressions I've used, or whether I had "fake" ROM and was using my neck/thoracic spine to compensate for my lack of shoulder extensions. However, to fix that problem I drew inspiration from the HS/PE5, HS/PE6, HS/PE7>iM and HS/PE8>iM videos.
All I do is lie on the floor face down. Then bring the bar up into shoulder extension. One thing to note is that when I did these progressions, I was already able to bring the bar to shoulder height by myself. So I don't know if it's a good idea to skip the 3 progressions I listed ahead and going directly to these exercises.
The order of progressions is exactly the same as the 3 progressions listed above as if you were standing.
Advancing in weight:
I started off with 2.5 lbs before jumping to 5 lbs. I've noticed that it is very difficult to do that, and you will certainly get cramps in the medial head of the triceps the first few times of working with a new weight. Especially if you're working by yourself, it's definitely better to start off with a lighter jump (even if you have a partner to reduce risk of injury.)
I also noticed that once you do the ROM to get your grip to shoulder width apart, the process once you move up in weight is much much faster.
Why should you not skip this mobility exercise?
One thing I've always noticed about HS/PE2>iM is how similar the shoulder position of this exercise is to the bottom position of Dips (referring to shoulder position only, not the elbows):
That has always made me think: Is it any wonder that many people in the general population (not referring specifically to GST population) has pain from Dips? - Is it simply from a lack of shoulder extension ROM, and then forcing that ROM in a strength exercise?
I've also noticed that the better I get in my shoulder extension ROM, the better I get in the "Rows" exercise from Foundation. What I mean is that, the better my ROM there is, the better I am at touching thumbs to shoulders in Rows.
Hopefully whoever that still have problems with this exercise, they can take something away from here: whether it's figuring out what issues they have, how to target specific flexibility places, supplementary exercises, etc., or even simply why they should keep persevering in this exercise, then at least I can say my post helped