Agree with you that Swain don't really hzve any slot left for a DFG, so much strong item for him...Maybe if your team got an overall high tankiness and lack damage.
Concerning linadry, i don't know. Like you Bystekhilcar , i am less and less excited by this item, except on few champions and i think swain is part of it. Your spells sinergyse amazingly well with the passive, u grab a little bunch of health and MPen wich is great, because i'm not grabing void a lot.
RoA seems to me like a must have on Swain; give you all the char needs to work. Even if, with Athene, u don't need so much mana, you will lack a lot of tankiness by skipping RoA. Sure u can rush Rilay, but u will have even less damage, wich might be a problem cause deathcap is not really a rush item on swain for me.