Just finishing playing this section and some comments.
My table saw a different approach to it as a PC death meant they were going to Garden of Morr for funeral anyway.
In any event, the placement of the holy water in the embalming area makes it not useful to good roleplayers. Good roleplayers won't be looting a holy place when as yet though they are concerned by whereabouts of priest they don't know there's a horde of zombies awaiting them. It would be more useful if there was another font of it just inside the Ossuary (encountered in rally step).
Depending on order of opening doors in mausoleum, Mourn is encountered before other chamber explored - so be prepared to accept luck of draw there.
Players are going to want risk shooting doorway at Mourn/Madriga. Even if only one line of sight can work and bad dice added because of obstacle. Be clear up front if they can or not and if they can I suggest making it a challenge die added. Mourn has low soak value and thus can be dropped pretty quickly by good attacks. Another option is to have him holding Father Grabbe up as he drains him (A) PCs see it more clearly than with body in coffin; (B) explains he's using Father Grabbe as a shield while he drains.
Just some general thoughts.