I think Saffold is first up at LT, so I could see it being either Arkin or Murphy, depends on who the coaches like better.
I would say you are 100% correct on the Saffold being the top OLT on this roster. In fact Saffold has played that role since Fisher arrived. But if you release Murphy you now have no other OL'ers on this team who can play OLT other than Saffold who many here think will start @ OG. Not having another OL'er on your team who can emergency stop gap @ OLT is not sound roster management.
On the other hand if you release Arkin who is a back up to both OG posts & emergency center your not in the same predicament because Boudreau has other reserve OL'ers who can easily take the place of his skill-set like Donnal, Wichmann or even Rhaney who all would be reserves if Saffold starts @ OLG.
To me its a easy pick here Murphy stays & Arkin would be released. It not like there would be many ball-clubs ready to pounce & sign David Arkin. Good chance David will be available later if his services are once again needed.