The Wight Fortress and Cragrock towers are rare (underground tar pools are also rare but becouse its such a good tar beast farming location i undertstand its rare) i have traveled over 10000 blocks from my portal tree to find one marsh with 2 wight fortresses (to be clear i wasnt looking for a marsh with 2 wight fortresses i was looking for a wight fortress i just found a marsh that happen to have 2 wight fortresses) and if the wiki is true and this structure is going to be critical to progression if it doesnt change when it does become critical itll be pretty messed up

what is less rare are lore scraps my suggestion is that lore scarps for wight fortress and cragrock towers cordinates or map will be displayed on a lore scarp for the neartest structure

cordinates or maps for the other structures are not needed if you ask me as they are less significent in their importance or rarity

Note: my coding knowlage is non-exisiting what i do know is that the /locate command only supports vannila structures and you cant add modded ones so if the structure map behaves like so you may not be able to use the same code as vanila and have to make a new locating system of your own AGAIN non-exisiting coding knowlage take this note with chunks of salt

I like your idea of obtaining a way to locate those structures from lore scraps or maybe also scrolls. Would defintiely be nice to have but with the way the structures are implemented right now it's unfortunately not possible. And that won't change at least until we have to rewrite worldgen again for 1.13.

Could also be pre-plotted Amate Maps you can find, which have the structure already labeled on it with its map icon.