No, Arawn wouldn't resurrect him since that would require getting personally involved in Creation again. Sucellus did resurrect himself after gaining the precept of Life, and even then he was only able to after the 7 pieces of his body were uncovered and brought back together in his tomb. I imagine that he had to rely on mortals to do that (probably the Ljosalfar, who were being led by Cernunnos, perhaps with help from human druids too), and that this was likely a quest nearly as epic as Kylorin slaying Mulcarn.
Sucellus didn't die the same way Mulcarn did. No single god has the power to destroy another god; only The Godslaer can do that, and that probably would only work against those who personally swore to be vulnerable to it. Mulcarn could not truly defeat Sucellus, so he broke his body up into 7 pieces and buried them far apart to prevent them from rejoining and healing. With his body temporarily defunct, Sucellus's soul fled to the Unerworld just like the immortal souls of mortals do. Inflicting this type of death on a fellow god is apparently well within the power of a single god, so there would be little point in creating a special weapon that kills in the same limited way. No, I'm thinkign that the death by the Godslayer is an absolute, final death. From what I gather, Mulcarn was probably destroyed soul and all. If his soul does remain, it is likely not to be found in the known planes of Creation, and likely beyond even the Metaverse that Ceridwen knows. Only The One really knows the fate of his soul, and only The One could save it. (Even if a soul is destroyed completely, I'm sure The One would be able to restore it, if only to let it stand accountable when he returns.)