I come for a Baptist background so am weak on Catholic info, but I never thought about it taking weeks to months ... That puts it into a new light. Heck that in and of itself could be a series of adventures. Trying to find the items that would weaken the specific entity . While the older more experienced priest (who is a bit frailer and not able to do active adventuring) with assistants hold the fort down and keep the spirit contained and suppressed a bit, the younger (adventuring) priest hast to go find some items that are tied to the history of the possessing spirit, or the chalice used on it the last time it was cast out 100 years ago, which will more easily weaken it this time.
I do like the idea of having to chip away at it, at least for the bigger meaner darknessess. It will be easy enough to cast out a ghost or the like, or a demon fueled by basic corrupt passions. However, going up against something more evil and powerful may have to be an incremental issue. At least it will have to be if you want the possessed person to survive the ordeal. If not you could contain the spirit in a circle, off the victim, and then banish the demon back to the dark pits of hell . Somehow I think killing off the victim though would have negative continuances for the priest tough
Also, when the exorcism is started how will the priest know if it is an easy to dislodge and banish passion spirit, or a greater more dangerous legion demon(s)? If they aren't careful, rush their prep, or bolo some of their skill tests before starting the rite, they could be in over their head, which makes the circle of protection even more vital !
Nice on the 'blessed silver and blessed lambs wool' I may have thought about some blessed silver interments on my own, but would have never thought about the lambs wool, and that is a perfect item to include. What better to tie the victim down with than triple strand wool ropes made from year old pure white lamb's wool - even better if the lambs were sacrificed by a Jewish priest - ... then the victim topped with a goat hair belt made for the hair of a goat used as a 'scape goat' ... I may need to do some more research on old testament sacrifices and see what would be useful as items in the rite