The time factor is interesting to contemplate. I'd also recommend bearfoil go looking for satellite images of the BP oil spill and the area around the rig shortly after the disaster. The oil coming up from 5000' was spread around an area of as much as 5000' in radius judging from the size of the ships trying to control the spill as it got to the surface. With AF447 we're talking about 15000' give or take. And we're talking about a lighter fraction which would rise quicker. So I'd expect pretty much the same roughly oval or fan shaped dispersion from fuel released from AF447 from the bottom, if any was. Of course, the fan shape would be filled in between its legs.
One other thing to consider is "why was the plane THERE rather than somewhere 'more logical?'"
A look underneath that spill may be in order. But, I suspect we're making more of it than exists and stretching too far for explanations. You're making some good points. I made a point worth checking but probably not entirely germaine to a solution for AF447 so you could see someone else what I perceive you doing. I certainly did not cross all the Ts and dot all the Is in the discussion of 121.5 and I stretched a little a times by way of demonstration. (Please forgive me guys.)
And regarding roll rate, as long as the roll itself stays under maybe 30 degrees to 45 degrees either way the antenna won't care. It's beam width is rather wide at the 3 dB (half power) points. It MIGHT matter if something else already made the path loss close to allowed margins. My INMARSAT experience is with the M equipment, not aircraft equipment. Among the INMARSAT specifications there are words about transmitter power management and minimum user equipment maximum power capabilities. M had pretty significant margins. From the West Coast of the US I was able to use the Atlantic satellite for testing quite nicely. It was about 15 degrees above the horizon so I was in a self interference zone from multipath signals bouncing off the ground. Margins were sufficient to make the link work even in pretty dreadful weather. (Yes, it does rain in Torrance California sometimes. My poor Beetle nearly floated away from one storm.)
{^_^}