I'm not sure I buy into the Marfans' angle. That theory has kind of fallen by the wayside, actually. There's little chance Akhenaten ever resembled the strange forms of the statuary and reliefs from the early Amarna Period--it was probably more of an artistic convention employed to established a physical sense of divinity among the Akhenaten family. And later on in the Amarna Period the statuary and reliefs of the royal family present them as much more conventional in appearance.
Also, the whole inbreeding issue tends to be overblown. No single royal family was ever in place long enough to produce severe conginetal defects from interbreeding. And even if the Tuthmosid line of rulers down to Akhenaten did inbreed more than was the norm for even the royals (and there's no evidence they did), Tut's mother, whether she was Nefertiti or Kiya or someone else altogether, likely had none of the genetic "contamination" that the father's side may have had.
Your weak heart argument is interesting, though. Of all the theories presented (and I used to be
a proponent of the assassination theory), I think the gangrenous infection is at present the most plausible, based on the evidence presented by the CT scans. Such an infection could well have damaged cardiac tissues and led to heart failure.