Tim provided a broad response to my general questions about youth development. His response can be found in full.
A lot of the 'solutions' to youth development depend hugely on the club vision and philosophy. To me, the overriding conclusion that comes from academic research is that there is no one-size-fits-all model. There isn't even an one-size-fits-all for individual player development or any particular coaching approach. I think - and I just think, there is nothing to support this beyond my own opinion - that the keys in youth development are flexibility and adaptability. What works for one player won't necessarily work for another. Loans will work for some, staying in house works for others. As long as there is a clear, evidence-based approach (i.e., not doing things because it's the way we've always done it) to decision-making by those who work in youth development, and that the decisions made aligns with the vision and philosophy for that players development, then the solutions we come up with should and will always be different on a player-by-player basis according to the vision and philosophy of that particular youth development program.
As a comparison, two practical examples of this is Chelsea and Tottenham. Without being privy to any internal details, and basing this purely off what I see as a fan, Chelsea's approach appears to be geared around developing players to be the best they can be, regardless of what that 'best' looks like. For the majority of players, this involves exposing them to [hypothetically] regular first-team football at a level appropriate to their skill and development. I don't think the club particularly cares what the player looks like and how they play (in the sense they don't mind if they produce a Kanté or a Fàbregas, as long as they develop the player according to their potential). Then, if they develop to be able to play in the Chelsea first team, they'll keep them, but if not, they'll sell them for a tidy profit. Chelsea still produces players to the same quality as any other academy or club, there is just a different philosophy in terms of what the end goal is for the players, and the process of reaching that end goal.
By comparison, Tottenham do it completely different under Pochettino. It's been interesting this year to hear him speak a number of times about how players with potential to make the Tottenham first-team squad in the future are deliberately kept in-house so they can train under him and develop to fit the very specific profile of player that the head coach wants. It's a completely different approach. But it's not necessarily better or worse to Chelsea, it's just different. To my mind, both clubs are producing similar level players. They just have different beliefs about the type of player that should be produced, and therefore, the process of developing that player. Pleasingly, the decision-making within each club's youth development program is, as far I can see, consistent according to their own vision.
Therefore, a lot of the other questions you ask are all answered by the question - "what is the club's vision and philosophy to youth development?". You ask what positions are easiest/hardest to play at a first team level. Well, that depends on the philosophy. At Chelsea, it depends on the manager at the time, and their style of play. That, of course, is ever-changing. At Tottenham, though, the easiest/hardest position to play is whatever you assume to be the easiest/hardest in Pochettino's style, because that is what the youth program is developing them to play in.
Speaking more generally, I think the hardest positions to break into at a first team level is anywhere in attack, and particularly creative players. There is a developmental problem in that we often see young players who try to dribble past everybody, or attempt outlandish things, as 'hogs' or 'selfish'. But that is the exact kind of player you want at a high leve, isn't it?. Messi and Ronaldo are massive, selfish hogs. They just happen to be really good at being massive, selfish hogs. But at a younger level, we tend to coach those attributes out of players by labelling them as such. But ultimately, isn't that the exact kind of player we'd love to produce? Of course, there are hogs who are just that - hogs. But you also have the hogs who have the ability to be very, very special players, and we tend to do everything but encourage their development.
This all ties in nicely to answer your questions about what I wish the public knew about coaching youngsters, or what is undervalued. For me, the main thing is that it's hard. Not in the sense that the actual coaching is difficult - my favourite part of the week is being out on the training pitch and interacting with players, delivering sessions. That's easy. The hard part is knowing whether what we're doing is the right thing in the long-term, particularly for the development of the players as players (not necessarily people). The human side of coaching - teaching players the value of teamwork, communication, respect etc - will always be similar, and important, but it is the technical/tactical/physical side I find difficult. Is teaching a player how to play out from the back important if we have a generational tactical shift away from that sort of approach? Is creating highly technical players vital if Premier League coaches start to emphasise more physical styles of play? It sounds silly, but we've just had a massive philosophical shift towards one end of the spectrum, and we can't rule out football going back the other way. You couldn't have predicted that ten years ago when the coaches of that time were producing the players we see now, so how can we pretend we can see ten years into the future from now, when the players we're currently developing are [hopefully] playing at a high level? You can't, and that's where my concern lies. A lot of clubs have a vision and philosophy based upon what was successful in the past five years, when we should really be creating a vision and philosophy for what will be successful five-ten years from now.