Great questions (hah..and I think you've learned by now that I have no problem with lengthy explanations

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GLH is certainly map dependent for sure. For example, it is generally pretty much useless on Pangaea maps, though I've seen rare cases where it could be useful. On watery maps though it is great as you usually have a lot of coastal cities anyway. Fractal sometimes yes/sometimes not as Fractal is unpredictable in what you get (I play this map personally a lot by the way). Fractal can produce pangaea like situations sometimes.
GLH is great in these circumstances regardless of iso or not. But iso situation it can be very helpful as it boosts your internal trade routes since you will not have foreign trade routes for a long time. And if you have nearby islands to settle in iso that will give you boosted overseas trade routes with the island city. (someday you will see this in practice). edit: As krikav said the decision in iso is relative to actual coastal city viability..sometimes your inner terrain is just better for say cottages and there is not much seafood.
Here though you have actually met some folks and see just how even better GLH is as you get those juicy foreign trade routes..3 commerce at the moment. Think of those trade routes like having developed cottages inside your cities..they both give commerce.
Mids is great indeed for Representation for boosted specialists and, of course, increased happy cap. Also opens up other civics that you might use in certain situation like Police State for faster army, but..yeah..Rep is definitely the main goal early with Mids. It is an expensive early wonder though, so usually not considered unless you have access to stone early or are IND..and you have both here which just makes you scream MIDS!!!
As for economy, we don't really discuss CE vs. SE these days but rather go with what we call the hybrid economy. The point here is that cottaging is in no way going to preclude using specialists (namely scientists early). Fairly recently, a series of articles called "Civ Illustrated #..." were put together by some good players. There are four of them addressing certain topics and really these are the only valuable articles of note in recent years...most are old and generally outdated. Anyway, one of them address this topic a bit and can be found here:
https://forums.civfanatics.com/threads/civ-illustrated-4-hybrid-economy.548667/
(Note: that is in the Strategy Articles subforum to this very subforum ...also check out "Know your Enemy" which is a great guide on Ai diplo and coding which you can always reference to understand what leaders prioritize. You can find it easily in that subforum)
As for Stonehenge, I would not say I never build it, but the key thing I want you to understand is that just simply building it is taking hammers away that could be better used expanding your empire early. Monuments as you have learned are not as valuable as you think. You get around this by simply making better settling decisions. there are times where you simply have to have a monument like to grab that seafood swimming to far off shore. Or like in Elie so we can grab the better tiles as we made the decision to get stone online asap. And then you have Creative leaders.....
And yes, polluting GP pools can be a problem especially as your early to mid focus on great people are Great Scientists for an academy and later bulb strategies...we'll get to that later.
I think you do want a library in Helio . I still considered you a bit in expansion phase though. I was suggesting take the OF hammers from the gran whip into a settler ..you can then grow a bit by queuing a Library which I might later 2 or 3 pop whip there. Or you could just let Helio grow for now ..after granary ..and 3 pop the library in short order at size 6.
With GLH though I like to focus on getting all the good coastal spots running asap so I'd still like at least 3 more settlers in the near future. All those cities will be a huge boost to your overall commerce and thus research potential.