Agree with most of your points. But there are a few that I personally have some conservations.
(1) Not everyone knew the team was in decline, not even today. Many still thought Ferdinand and Vidic were still in their prime, despite being so error prone.
(2) I don't think van Gaal was stabilizing the team. What he did was just kicking everyone out and replacing them with inferior options.
(3) He had laid a terrible foundation for his successor. The squad was criminally thin and many of the players were simply not good enough.
(4) His youth policy had done more harm than good, to the team and to the youngsters. Refer to the OP for more details.
(5) There was an actual need to bring in Ibrahimovic at that time. Rashford and Martial were on the rise but they were not mature and consistent enough to lead the line. It made sense to sign a veteran striker so that quality could be maintained without sacrificing Rashford and Martial. He was crucial to our cup success, but his injury cut his spell at United short and the whole plan had fallen through.
I remember saying to friends in the spring of 2013 that I thought Utd would still win the EPL but that I'd be surprised if we won by more than 3 or 4 points. We only won three games out of eight from the loss to City in April, and I was astonished we won eventually by 11 points - it was a tribute to SAF's ability to keep winning as well as a failure by City to take advantage. Rio, amongst others, has gone on record that the team knew it was in need of refreshment, so, yes, some people still criticise Moyes for turning a winning team into one that came seventh, but I still think the signs were there at the time.
As far as LVG goes, this is where I think we still don't really take a retrospective view. Football, particularly at the level of EPL, is, I guess, mostly a short term matter - most people want to see trophies and results
now, particularly if they are shelling out for a season ticket or satellite or cable subscription. Moyes had to follow a manager who had managed the side for 26 years and almost everything led back to SAF. If there is a criticism to be levelled at him, it is the touch of old-style football cronyism that left a vacuum after he retired. Regardless of how much was Moyes's fault, he failed and left a club puzzled by the extent of its fall from the top spot. The big debate at the time was whether we should appoint someone in the Utd tradition (e.g. give Giggs the job) or go for an experienced manager with a track record. LVG provided the sense of stability, that we wouldn't lurch from one system to another in the way Moyes had in his 10 months.
I agree that it failed eventually - as I say, it was taking the club in a direction it didn't want to go. I would still rather have given him his third season, though, rather than appoint Mourinho, whose appointment I always thought would end in tears. In ranking the three managers, any harm LVG's style left was way less damaging than the toxicity of Mourinho and the constant reminders that it was all about him. He wouldn't have left a title-winning team after three seasons but he would have left a calmer, more structured side for a successor to move forward.
Regarding the squad, he said from the start that he believed in working with a small squad, with players who knew the manager trusted them and without players becoming frustrated and disruptive because they didn't play enough games, and that when he needed cover he would use the second team. The club knew what they were getting when they appointed him. Using youth players is a longer-term business - it involves trying players out and giving them the time to develop, as very few are fully formed or exceptional players from the start - a problem with have now, think of how McTominay was derided as "not good enough" and just a Mourinho puppet. Some of them will fail, as we saw with some of the names you quote, although some could have succeeded given more time.
If I have a criticism of his youth policy, it is that he discarded youth players too quickly if they had a bad game (look at how many were damaged by the MK Dons match). However, the principle is better than Mourinho's policy of having two specialists for each position and then pitching them against each other - I'm all in favour of players competing for their place, but not the Mourinho tactic of setting them at each other's throats, and it's just too expensive spending huge sums of money on players, only to discard them soon afterwards. It effectively renders your youth system pointless.
I would agree some of his signings weren't good enough - as I say, they tended to be very functional, not counting Di Maria and Falcao, who were for me very untypical of LVG. He did sign Martial and Romero, Shaw and Herrera belong to his time (accepting they had already been agreed) and Memphis and Blind are not bad players, in Memphis's case more one of his own personal issues rather than a bad signing by LVG. You could level the same accusation at Mourinho, e.g. Mkhitaryan, Sánchez, Lukaku, Bailly, and the big question of how good Pogba is for Utd as a team. I had no issue with Ibra personally and he brought a feel-good factor but I still think it had more to do with Mourinho making a statement about himself and his low regard for Martial and Rashford.