Here's my take on this one. Hope it doesn't lead you up the garden path.
Your collapsing hose can only be due to a partial vacuum inside the system, created as the temperature falls and the air in the system contracts.
The system is almost fully sealed and you do have air in it at all times, even when all air locks have been eliminated. This is the air in the expansion vessel and it needs to be there.
Since you mentioned the collapsing hose, I went and looked at my 164. The hose is not collapsed at all, merely "relaxed" when cold. If that is the correct state of affairs, this means that in normal conditions, any partial vacuum is released as the system cools and the only place this can happen is the pressure cap on the expansion vessel. On mine, I can blow and suck on it when cold from the inside and it seems to flow more freely out of the cap than in but nevertheless in both directions. Doing the same on the vent tube has the same effect. My GTA is similar (but no vent tube). Suggest that you check yours, especially for air admittance since it would appear that expansion has expelled some air to later cause the partial vacuum. My diagnosis is faulty pressure cap
While I'm here, I don't think that the hose would necessarily be deteriorating to the point that failure was imminent, but it would be prudent to replace it if it's at all old.
When I refilled mine from scratch, I did it with the thermostat cover off and the expansion tank out to start with. I first poured coolant into the expansion tank connection, always very slowly so as not to fill the pipe with a "plug" of water. When coolant emerged (thermostat housing first IIRC), I refitted the cover and removed the bleed plug. More filling. When coolant next emerged - not sure if from bleed hole or if the open pipe for the expansiopn vessel topped out first - fit next bit and carry on. Once the expansion vessel was fitted, carried on filling there, very slowly as before, refitting hoses as and when needed. No air locks at all. A very little air bled out after starting. The heater should self bleed as in common waith many modern heaters it does not have a water valve IIRC, relying instead on the control of air flow over the heater matrix.
Think I might have laboured the point a bit. Sorry and hope this helps.