Alright, folks... Glow plug exchange complete at an engine workshop, all brand new now except number #6, which is stuck. And the most difficult one to get access into.
Guys refused to turn because they wanted a confirmation from me due to the plug possibly braking in the process.
First estimate was that they can probably remove it with drilling techniques, which would result in a moderate repair cost. This is the option that realizes in most cases.
The second, and worst, estimate is that, should the plug get broken or should the tip fall into the cylinder, they have to drop down the engine in order to remove the cylinder head, in which case they definitely can remove the plug, but this option is very costly.
What surprises me most is that the engine needs to be dropped even in case of the V6. This is one main reason why I stayed away from the V10 when purchasing my car to avoid costly engine removals...
I decided to hold back for now and think for possible other repair options, if any exist. I have read about various magnetic fluids/oils that you should spray on the neck of the plug while juggling it back and forth using small steps to get the soot off which keeps the plug stuck from the inside, but this seems like a lengthy and cumbersome operation to take on a vehicle that is in constant daily use.
Any suggestions from fellow T-Reggers? I have a very strong love-hate -relationship with my vehicle, and now the scale is leaning towards the hate side again...