No, save your money, because you clearly state that it's tight.
If it was WTP to business then totally different scenario.
Outbound it's a day flight, that's easy enough. Return is shorter, thanks to jetstream, and so just expect a not-so-great night (great practice for when newborn is along).
Row 51 is near the toilets, so could be handy, but as it's near the rear of the plane you'll be more or less last to deplane and that will add time to immigration queues too.
There may be worth in requesting assistance through manage my booking (MMB), say if your wife can't stand for long periods, as this may also expedite your immigration experience (I've only ever done this through injury, not pregnancy, mind, so not sure it would work!)