>>96728186
Seems kinda contrived.

If it's important enough to have 100-200 astartes tied up for 100-200 years, and still fail to completely purge, then Helbreacht is going to give a shit and order reinforcements there or other chapters are going to get interested.

May as well just have a chapter founded from BT stock as a warden for that region of space, with good relations with them so they send a few sword brethren squads to instruct and aid them. Kinda like a Novamarines/Dark Sons deal. Defining the enemy they fight and the local customs they take on is more important for the chapters identity, keep the BT inheritance simple.