That would be an excellent explanation for why Marsh got both Allo and Feru Atuim spikes from Ruin. There might even have been a spike for Feruchemical Nicrosil as well to give him even more time to look.
I would say that one of the reasons Saze would have for giving marsh the Atium he needs to live for a long time is to keep in balance, remember that there's more Preservation in people than there is Ruin, so if Saze wants to keep in balance he needs to get rid of some of ruin's power. Giving atium to Marsh seems to kill two birds with one stone, as it were.