Wrt a Bleeds Off Takeoff... it might be pointed out that the initial two items of Boeing's B737 'After Takeoff Checklist' are "Engine bleeds ON" and "Packs AUTO" and it's written like that precisely to address the 'Bleeds Off' takeoff scenario, such that the air-conditioning system is reset at the earliest opportunity (on the assumption that a 'bleeds off' takeoff has been conducted) wherein the after takeoff checklist is typically conducted soon after the flaps have been retracted to 'Up' (veritably PM = "Flaps up; Lights out" --> PF "After Takeoff Checklist") and, in the bleeds off takeoff scenario, before the 'After Takeoff Checklist' can be completed, it must invariably be preceded by the instructions contained within Boeing's B737 FCOM / Supplementary Procedures / Air Systems / No Engine Bleed Takeoff And Landing / After Takeoff.
So, it'd be pretty gash to do a bleeds off takeoff and then somehow forget to reset the air system (i.e. when it's the first item on the after takeoff checklist) which is all assuming that that's what might have happened and about which there is no evidence to support that it did.
Wrt 'fumes'. Air distribution throughout the cabin of a B737 is pretty much uniform, wherein (even taking account of the injection of 'Trim Air' into the forward & aft cabin air supply ducts) - and with the recirculation system in operation - cabin air pretty quickly gets sent back through the Mixing Manifold (where it's mixed with air from other sections of the cabin) before being re-fed back into the cabin again. Accordingly, air quality in the aft cabin is fundamentally the same as the air quality in the forward cabin and air delivery (via the risers, thence into the overhead distribution ducts) is, again, pretty much uniform throughout the forward & aft cabin zones, such that cabin air delivery in the very rear seats is the same air thats being delivered out of the louvres 5, 10, 15 rows in front. Accordingly, I'm far from convinced about a 'fumes' cause.
Wrt "the purser tells you that a passenger is having an epileptic fit"... the last time the purser said that to me, the pax who was having what, for all intents & purposes, appeared to be a 'fit', was in fact suffering the effects of a massive overdose of cocaine (latterly, it turned out that the pax was a drug 'mule' and one of the packets of cocaine that they'd swallowed had burst inside of them). In spite of the cabin crew's best endeavours to keep that pax alive, the pax was pronounced DOA (at the airport we diverted to). There then followed a couple of hours on the ground, whilst a Dr. and a Coroner were located (a Dr. to certify the pax 'clinically dead' and Coroner to certify them 'legally dead'), thereafter the body was off-loaded and we then carried on to our destination. Hey ho.
Nonetheless, it's a tricky call. Some might say "If in doubt, then there's no doubt" (i.e. put it on the ground and sort the problem out there)... but we weren't there and we're also not privy to all the facts.