I'm interested in obtaining Mulcibar's Scoria, and am willing to trade 1 for 1 with Umbral Marrows, up to 3.
ZNM is fairly easy to do solo aside from a select few, but a 2-3 player group should have no problem obtaining Pandemonium Keys, and this may be an option for you to obtain Umbral Marrow easier, or to make money as Umbral Marrows sell regularly at a high price tag. Of course you would have to kill Pandemonium Warden to obtain the Scoria, which can be done easily with a shout group, but if you cannot get a group to defeat Pandemonium Warden for you, I may be willing to purchase the keys from you.
Please contact me if you are interested or if you need any information on how to effectively do the ZNM's so that you may tackle this offer.