it's a long term plan for me as well. Something useful to put seeker actions in after we've got the basics down.
And it would be a backup for the webway, mostly.
If the webway narrows to the point that would strand the craftworld, we would still have a way to cross galactic distances.
As for leaving the fleet behind, they should still have access to a narrowed down webway, so they could travel to the craftworld's destination in advance.
The craftworld would only be unescorted while traveling faster than light in the materium.
like i said, i don't think it's a difficult technology to reseach. Manipulating the webway is not mutually exclusive with having a better backup ftl, but it is a very difficult technology tree to research, especially without a sample to work off.