What are the electrical requirements for magsafe charging?
Is it safe (1) to omit the initial high impedance 6 V or 3 V and
  simply apply "14,5V to 18,5V on Magsafe 1 and up to 20V on Magsafe 2"?
  Do I have to match the voltage of the power supply reported by the
  cable connector (16.5 V)?


The "initial high impedance 6V or 3V" is handled by the power supply that sends the 3/6V until it detects a "power line low" condition (the MagSafe is connected) at which point it sends the full 16-20V DC

Do you need this initial 3/6V?  No.  What you need to do is supply clean, stable 16-20V DC to the MagSafe cable (shielding to negative, white inner to positive) because that's what's required to power your Mac.  


  This risk seems to be why the PS only turns on full power after it senses 40 kOhm across the power pins for 1 s.  Is there a risk of shorting the pins when the plug is not properly seated? In that case I'd apply power after nicely seating the plug.


Yes, there's a risk - a low one.  It's extremely difficult to short the pins on a MagSafe connector.  I'm not saying it's impossible, but the design makes it so it's extremely difficult to do so.  However, if you do short the points, the damage will most likely be minimal (you'll blow the MagSafe DC-In board and not the whole logic board), but in my experience, it's not worth it.

What I have done is connect a MagSafe to a switched AC adapter "wall wart" that supplied 18V at 3A to my MagSafe and powered my "benchtop" MagSafe diagnostic power supply.  I did it this way so I could insert a multi meter in line so I could measure amperage draw.  I've never shorted pins and this worked great.

Standard disclaimer:  Don't try this at home.  I only play a professional on TV.