Chauffeur drives should be bookable in MMB on EK's website. For a couple, definitely go for the middle seats. As for the shower, sure why not, ask for a slot the moment you get escorted to your seat upon boarding to have better chances at snapping the most convenient slot.
Can't comment on lounge food in JFK, but for what's worth go for it in DXB if you're on the redeye EK384/374 services, or save it for onboard if it's the EK372 service.
As for 'maximizing' the experience in F, I got this comment from another website: do a dummy run in economy the week before.