Just thinking about the canal. A ship canal is a vast amount of work to build. Ocean going ships have much deeper draughts than canal barges do, which makes everything enormously harder. It would probably be literally orders of magnitudes harder to make a canal through which the Empire's warships can pass than one through which barges can traverse - and almost all the economic benefit, particularly for Kislev would be realised from having a barge based connection.
Making a deeper and wider canal isn't just about digging a bigger hole, it's that the locks require vastly higher standards of engineering due to the geometry (cubic volume of lock, squared area of lock gate, and linear thickness of materials everything is made from).
There's also the question about whether the rivers in question are navigable for warships rather than barges that far inland. The difference between the two should be significant. Many rivers are navigable for barges for hundreds of miles where a sea going ship couldn't pass.
Ship canals generally are megaprojects requiring twentieth century technology, unlike barge canals.
If we ask for a canal, building one for barges for trade and military logistics is quite reasonable and should be the work of years or a decade or so. A ship canal seems like it should be a much bigger job.
There is no reason why they would have lost information that general, their imperial core never fell and dark elf raids are well... raids, not the kind of stuff that wipes out knowledge of infrastructure. For comparison the much smaller Laurelorn has a map that contains Albion, much older and much less relevant to them.
As far as I can understand it, they did though. As far as I can construct from the various army books, the history of the wood elves goes something like this:
Golden Age elves (re)discover Athel Loren and finding it a scary place filled with wood daemons, build a network of magical standing stones to pen them and the trees they inhabit in.
The elves settle the coasts, plains and other forests of what will become Bretonnia. Some of them settle permanently in cities and establish farms, while others (the future Wild Rider kindreds) appear to be at least semi-nomadic, Ellyrian colonists who occupy the plains with their herds.
The war with the dwarves happen, many elven cities are burned and then many elves are recalled home to fight Malkeith's later invasion.
Immediately post-war, there is the massive climatic disruption caused when the Old Ones fiddle with tectonic plates. This is noted in dwarven sources to have destroyed their above ground herds and prevented their crops from growing (potentially for years). Presumably it hit the remaining elves in the future Bretonnia as well. Some of these elves settle the edges of Athel Loren.
After several hundred years of holding on to the land despite these set backs, the plains elves (see previously mentioned future Wild Riders) and those living around the edges of Athel Loren are finally defeated by a vast Waaagh of greenskins. This is the same period as the Time of Woe when the dwarves are also under assault by the greenskins, so presumably this would just be the latest assault by them.
The defeated elves, left with no other choice, flee deep into Athel Loren where the leadership makes a deal with the tree spirits for mutual survival. Together they defeat the Waaagh when it comes after the elves into the forest, and the Asrai come into existence as a distinct group.
About a century later the Bretonni cross the Grey Mountains and occupy the lands 'recently' vacated by the elves.
Many centuries after that, when the inhabitants of those lands were once more threatened by a huge Waaagh, the locals once more cut a deal with a powerful magical entity to survive. This time it was the Lady of the Lake, and they did well enough/could recover fast enough to retain control of the territory.
Essentially, if this is correct, the elven refugees who became the Asrai may well have lost a lot of know she when they fled to Athel Loren.
Conversely, when the Bretonni arrived they would presumably have occupied elven settlements that has been left relatively recently. That means that the Bretonni priest-kings (assuming they followed the same Pre-Tilean practices the other similar tribes they left to the east did), may have had access to significantly better preserved elven sites than most others, and had the opportunity to do what the Belthani had with learning from nexuses, etc. The Damsels presumably would have inherited that,