The lore we are looking for, if it exists must be very closely held we we have found no trace of it in Grey Order records. We found more indication of Druid secrets in the library of the Jades and we are not a Jade.
Although I generally agree with your points, something to consider is that the Jades and druids and Hedgewise and Greys are in very different situations.
From what we know, the druids were effectively another nation. They may have lived inside the Empire's external borders, but they were like the Eonir forest born (and, if Boney is using that part of lore, the other much smaller remnants of elven colonies living in other forests). They were a mostly separate ethnicity of forest dwelling nomads that had lived mostly isolated lives for thousands of years, since the pre-Imperial tribes displaced their Belthani ancestors from the higher value land around the rivers. I'd put decent odds on them speaking their own language and having their own pictographic written language. They'd be the objects of Imperial, beastmen, and forest goblin aggression, but they persevered. They seem to have gone through such selection pressure that they were all or mostly magic users to some degree.
That means there were lots of magic users all working and living together, in a good position to pass down their knowledge as there are lots of potential teachers and apprentices, and probably written records carved in hidden locations (although that's speculation). They were also unified enough that Teclis could gather and address them, and they seem to have retained one consistent culture without significant divergence, so they were able to keep in contact enough that if they moved culturally, they moved to together, which also reduces the chances of knowledge being lost.
Then Teclis appeared and did such a spectacularly good piece of diplomacy (probably rocking all the charisma and intelligence buff spells) that he convinced two thirds of the druids to join. Probably by convincing them that their 'Great Mother' wasn't a deity but an impersonal force, Ghyran, or at least that the Wind of Magic they thought was their Great Mother was actually distinct from her. The record of it we have suggests that this involved him doing something involving Waystones. Possibly he used them to assume direct control of their local flow of Ghyran in a way that convinced them that it wasn't a divine being because he could control it.
That meant that those two thirds of the druids that became the Jade College already had a long history of being an organisation of magic users with experience of living and working together as very closely connected 'institutions'.
The Hedgewise are very different. They don't seem to be a distinct ethnic/'national' group. Instead they were a social role that existed within the proto-Imperial tribes that were outcompeted and driven to the margins of society by the more successful Cults after the latter organised themselves on the Tilean model. This was probably not helped because their form of mediation between mortals and supernatural phenomena may have always focused on boundaries (although at a distance of thousands of years, it may also have been an adaption to the niche they were forced into.
The way the Hedgewise operated, as widely scattered individuals or master-apprentice pairs makes the retention of knowledge very challenging, even without thousands of years of persecution. Everything would pass through lineages, and a single broken link in a chain because of misfortune can result in knowledge being lost forever. This is made even worse by the difficulty of travel. While the druids seem to have been nomads and so managed to move to meet each other, the Hedgewise were sedentary, living on the edge of villages as part of imperial society. That means that if one village's Hedgewise dies without finaihsing thr education of their apprentice, it's hard for their peers to know that or to respond if they do. When you're a member of a persecuted profession travelling is even more dangerous than for regular people, particularly if that profession is a magical tradition uniquely dependent on specific tools and reagents your enemies can look for. The same applies to writing things down. The various varieties of Magick are their own languages, which a Witchhunters may be able to recognise.
Note; when I'm talking about links in the chain of passing on knowledge, we're looking at something in the range of three hundred generations. There's no wonder that the Hedgewise groups all worship different gods and have different origin myths. It's entirely possible, indeed, possibly even likely that at times Hedgewise magical traditions have died out and then reinvented a couple of generations later based on the legends and what scraps could be passed on by non-magical Hedgefolk, like how much neo-paganism is reconstructions.
This fragmentation may explain that when some Hedgefolk joined the early Grey College they couldn't dominate it as the druids did the Jade. Fewer Hedgewise joined and those that did were all from different places and so had different cultures and traditions. The druids had just been druids with a single solid identity coming from the outside and joining the Empire as a single body. The various Hedgewise were individual imperials, originally from different provinces with their own disparate provincial identity, dialects, and rivalries who had much less in common with each other. The druids moved as a community of magic users. The Hedgewise who moved moved as individuals each leaving their own separate non-magic using communities. Indeed, I doubt there was or is really Hedgewise communities in a meaningful sense, anymore than there are true communities of rural smiths or millers that transcend their village identity. Being Hedgewise, magically talented or not, is much more of a role within a community than a community itself, unlike the druids. There are just rural communities in a spectrum from the Hedgewise being able to operate openly, to operating clandestinely, to having no presence at all.
It's also worth considering which Hedgewise would join the Colleges. Unlike the druids, who seem to have come as full clans from elders to children, it seems likely that the Hedgewise most likely to defect to the Greys would have been the youngest and least powerful and knowledgable ones, as they'd be the last strongly settled in their rural communities. I think there's also a mention that Teclis was careful only to let people least likely to abuse Ulgu join the Grey College, which may have made him reluctant to admit older Hedgewise who were more likely to stick to their own agenda.
All together, it makes sense both that the Hedgewise would have retained a lot less of their original knowledge about Waystones and that fewer of those remaining secrets would have found their way in to the Grey College's library,
And that's not even to address the question of where the ancestors of the Hedgewise would have learned about Waystones in the first place. There's nothing I can see in their origin stories that particularly links them to the construction of Waystones, and the same thing that makes it hard for them to retain knowledge makes it hard for them to do the research required to learn it and then share and preserve it that more organised groups may have.