Where to start with death rattle?

One thing worth remembering is that what's often called "the death rattle" is the sound made in the transmission when the engine crankshaft rotation isn't steady enough to stay matched to the MGs, and the slight lash in the meshed gears gets clanked back and forth. It's a very alarming sound, which results whenever the engine runs roughly / misfires for any reason; the two possible reasons you've been focusing on so far are two out of many. Not all of the possible causes are major issues, so the term "death rattle" is kind of overwrought.

There are a few causes that are kind of well known. If there is just a brief rattle on first start, after sitting cold for a while, and the last use of the car was short enough the engine didn't fully warm up, then the cause of the rattle was probably condensed water collecting in small intake manifold passages. That particular problem has been known a long time: the first reports were from people driving the earliest "model year 2010" cars in fall 2009. The answer to that issue mostly boils down to: avoid those short uses where the engine doesn't warm up.

If there is misfiring only at mid-loads (not idle and not full throttle). there is probably clogging of the small EGR passages in the intake manifold. Only at mid-loads is EGR used. and so if there is misfiring only at the times EGR is used, probably the small passages are delivering it unevenly to the four ports.

If there is misfiring at idle, when there isn't supposed to be any EGR, one possibility could be an EGR valve that's not completely closing, but that would have to be checked along with many other usual suspects (plugs, etc.) for misfiring at idle.

The behavior usually associated with a head gasket problem will be a misfire right at first start that clears up fairly quickly, but happens consistently (not just occasionally after short non-warmup drives, like the condensed-water business).

 

This is typical of an early hg leak and will repeat occasionally as it slowly progresses over weeks or months. An egr clogging is more likely to cause stumbling and stalls if the egr valve is stuck open and do nothing if the system is clogged shut. In fact one of the best ways to eliminate the egr is to completely block egr flow.

The good news is a 2016v has updated pistons and rings and is not as likely to have a leaking hg.