Some references to your questions - very interesting ones by the way.
*** MEL ***
JAR-OPS 1.030
Defines the need for a MEL not less restrictif than the MMEL from the manufacturer.
JAR-OPS 1.085 (11)
The option to a crew member to deny a flight with an actual MEL or CDL item.
JAR-OPS 1.630 (a.2)
Defines the need to comply with a MEL before commencing a flight.
JAR-OPS 1.845
Same for the Navigational Equipment.
JAR-OPS 1.1045 (8.6)
Suggests that the operator needs to specify in his Operation Manual Part A how the MEL and CDL are to be used.
Common Sense: (Yes, I know you wanted facts... I simply never found that in the basic JAR-OPS - and do not yet have access to the full with IEM's and AMC's)
The JAR-OPS (as fas as I know) does not exactly specify when to use the MEL. Most operators I know allow the use of the MEL before the commencement of the flight - which is usually defined by: "the aircraft moving under it's own power for the purpose of a flight", while once in flight the manufacturers AFM comes into play. Try to search the IEM's and AMC's.
*** TOGA ***
EASA is currently the certification agency in Europe, equivalent to the FAR-25. found here:
http://www.easa.eu.int/home/index.html
EASA CS-25 AMC 25-13
Suggests that it depends only from the engine certification. In general as far as I know no time limit is imposed by the certification. Most limitations are either, noise-abatement or engine limitations. I am fully aware of the 5 min. time limit to some aircraft but do not have a better answer.
*** VMC ***
I'd not consider flying slower than VMCA in any circumstance except committed to land - and then it's in someone elses hand if your engine fails exactly in a low go-around at touchdown speed. (Only thing left to say - good luck or sh....t)
I'm currently flying the ATR Turboprop where the VMCA speed might be above the actual approach speed. In our operation in this case VMCA becomes the limiting speed and we do not go below that and use at least VMCA as approach speed. Makes sense,does it?
This may be useful. From the Jeppesen "Multi-Engine Pilot Manual" They tell you to reduce power on the operating engine and pitch to a lower angle of attack. One, not to loose directional control, the other not to stall!
If it happens on a demonstration of flight below Vmca and you are at 7'000 feet, fine. If it happens in approach at 150 feet AGL - pray that the tower is not on the bad side (the side the A/C will turn to if power is applied) and the field is flat and empty and the gear already down. If not - sh...t!
So - alway keep your speed up!
*** INFO ***
All information is given as is with no responsibility in any aspect to accuracy or sense or completeness. No liability can be derived or requested upon this post.