When the softener was in use before its removal was there any staining?
RO system will only remove one of the forms of Arsenic and not the other.
If the softener when it was working was removing the staining from the house, I would first put a softener back in and then test for the Arsenic in the water and find out what the level is, then if there is still some level of Arsenic research research and then some more research as to what level is good or bad... I have a challenge in taking what the Government says is good or bad but that is just me.
Then if the level is above what you would like, there is a media that is Arsenic only and use a point of use under the sink for the removal of the Arsenic.. the iron first has to be removed or it will not work.
Just my thoughts on the matter.