Just here to confirm that you absolutely can use her bonfire with or without Rosaria dead. Inability to use it is likely tied to Hazel (the slug in the room). Just killed Leonhard, have her soul, teleported there and can rest/teleport from there as per normal with Hazel dead. There's really no benefit to reviving her unless you simply want to. Granted, using the soul gives a paltry amount, so it just depends on whether you want the regen spell or not. I have no real use for it on this character and don't care about the small amount of souls, so I might as well resurrect her.