Thanks for this post and welcome to the EK forum. Two things that surprise me: didn't know they had corn flakes onboard, and this is one of the only instances I've seen of the first choice not being available in F, where generally EK is well regarded for not running out of options.
The shower is unfortunate, the plastic can be cold, one could probably ask the crew to increase the ambient temperature but one shouldn't need to. For what it's worth, 5 minutes of running water is a luxury I don't have in Cape Town where we are restricted by water restrictions to 90 second showers, so I can only partially empathize
I've recently treated myself on a few occasions to 4-minute showers, ssh, don't tell the mayor!
FUN FACT: While EK don't fly an A380 to CPT, the airport has published water restrictions in place requiring all aircraft to tanker their onboard water from their origin, i.e. no refills, wonder how this would affect the shower as I believe that EK has a limited dedicated water supply for these, so it probably depends on what's left over from the inbound flight.