Yeah, Reinbacher is a bit of a unique case. I'm not sure he goes 5th overall if Kloten doesn't get promoted. He could have completely destroyed the SL but the NL is obviously a far better league.
Obviously, an NL team could have bought him out of his contract if Kloten didn't get promoted but firstly that would have been unlikely to happen and secondly, he probably wouldn't have gotten the same opportunities elsewhere in the NL.
So you can definitely say that Reinbacher was lucky and in the right spot at the right time. Having said that, he earned everything he got and he was good enough to make the best out of his opportunities. He averaged 7 (!) minutes more in the NL compared to the year before in the SL. He doubled his ice time over the summer if you compare SL playoffs and the first NL games. Heck, even his now 36yo defense partner averaged several (I think it ended up being around 3:30) more in the NL. Reinbacher even managed to make the team vet better.
Kloten Coach Tomlinson is a smart man. He knew right away what he had in Reinbacher. His team is one of few in Switzerland's NL that doesn't have seemengly unlimited money to throw at players. So when they got promoted, he just went full yolo mode and relied heavily on Reinbacher pretty much from game 1 and never looked back. The rest is history. Reinbacher/Kellenberger played the toughest minutes and were still the lone plus players on the team. They, and the strong PP, were the reasons why Kloten avoided Playouts and made it into pre Playoffs.
So yeah, it's possible Reinbacher was lucky and is more of an under the radar pick if Kloten doesn't get promoted but firstly that doesn't make him a worse prospect and secondly, you gotta be good enough to capitalize on such a chance and Reinbacher was.
Regarding Switzerland, sadly you're right. The problem is that coaching is very poor. There's very few good Swiss born coaches and clubs have to hire foreigners. That, and the wealthy clubs not caring about prospect development, leads to rather poor junior leagues. To make the jump from juniors to NL is very tough and not all NL teams have a farm team or affiliation deals with SL clubs. In fact, EV Zug, the most successful club in Swiss hockey the last few years, withdrew their academy team which produced lots of prospects for them and other NL teams from the SL ahead of last season. So at the end of the day, Swiss junior leagues are too weak to still play there in your draft season but the jump to the NL proves to be too difficult for most kids especially since, when in doubt, they won't even get a chance. As a kid, you usually only get to play if you prove to be better than any vet the team has and could potentially sign. SL is a good option but those spots are limited as well and even though it should be, it's not a traditional development league so if your team doesn't have an affilitation it can be tough to get a fair chance there as well. Also, coaching isn't very good either. So sadly, I agree Switzerland isn't usually a great place for prospect development. Even the Swiss top prospects mostly leave the country at some point. It can still work out even for foreigners though but it's not a model that would work for everyone and having a Swiss playing licence (as you mentioned Reinbacher has) helps. A few recent prospects have played in Switzerland...even foreigners like Auston Matthews, Elvis Merzlikins, Brennan Othmann or Mason McTavish but you have to be in the right spot at the right time for this to work.