1. Vref is 1.3 times the stall speed in the approach configuration. It is a calculated minimum safe speed for the approach. Many operators will approach at speeds higher than this, eg Vref+10
Vth (I'm assuming you're talking about what I know as Vat - speed At Threshold) This speed is not so much of an "official" speed. However if the approach is not going to be flown at Vref, then the Vat speed may be discussed, for example "I'm going to fly the approach at Vref+10 and close the throttles early to slow reduce my Vat closer to Vref".
2. Actually, it stalls at the same EAS (Equivalent Air Speed) To explain this would take many pages. If you've got plenty of time on your hands then I suggest you research:
IAS (Indicated Air Speed) - Shown on the instrument
CAS (Calibrated Air Speed) - Corrected for position error
EAS (Equivalent Air Speed) - Corrected for position and Compressibility
TAS (True Air Speed) - Corrected for position, compressibility and density
3. An aircraft may be having an emergency, but not declare a "Mayday". Mayday means something very specific. It means "We require urgent priority to land"