My tuppence worth in response to your last post - only turn off current heading if on a runway centreline, it's an established procedure, any other supplements to that may cause us poor ATCOs confusion.
I would not bother with the "request heading" in the mayday call.... if you manage to squawk 7700, other sectors will see it and will coupled with the fact that they will see you descending rapidly, they will move their A/C out of the way if needs be.... on top of that, the controller you are in contact with should automatically be looking for some clear airspace as soon as they hear the words "emergency descent".
In the first few seconds of the emergency, the ATCO will get a bit of an adrenaline rush (though not as much as you!), and the 'request heading' may confuse them into thinking that you want directions to a specific point etc etc and may cause them to question you - the last thing you need!!
In busy airspace with a huge rate of descent,I would be very wary of TCAS - it is there, as always, as a last resort. TCAS can be extremely unreliable in azimuth and should never be used by pilots in this way, apart from when trying to acquire visual contact when you get a TA. Even then, caution must be used - you may become 'visual' with the wrong A/C!!
Pilots have, in the past, caused losses of separation because they did not believe the ATCO, whose traffic info seemed at odds with the TCAS display. The pilots subsequently caused erosions of separation, when in fact, the azimuth interpretation was wrong!!