As for the Second Proposition, i tought id make an illustrated description because it is really important,
So the goal is to separate this threshold into vascular and Bone
To do this, the method i found is to use editable=inside that layer to paint seeding regions in two new segments
Then hide the original threshold, and do a growfromseeds with editable=Inside the hidden layer, to get this:
The problem here as you see is that grow from seeds defines cropping region limited to the boundaries of the seeds to speed up the processing so the solution was to add a new segment and use editable= everywhere to draw the boundaries of the volume as shown here
Then proceed the same way with growfromseeds and editable=inside the hidden layer to get the final result:
What i would suggest is either adding an option in grow from seeds module to extend the cropping zone to even the hidden layers or to a specific layer
OR to make a separate module that wil take as inputs the segment to split and the two seeding segments and use grow from seeds as usual, taking the whole segment into account.
This is the feature i used the most and it could get way simpler,
Hope this Makes any sense!