If the ambient temperature is already close to 500°C (at a depth of around 13 km), I cannot see how you want to work with a liquid - and there is (NOT YET) no drill at all working and generating additional heat! And your great diamond drill bit will be worn out in no time at such a temperature. The idea with the bucket will certainly work up to several hundred meters - if the drill and rods have been removed from the borehole beforehand - I am curious to see what the client says about a drilling system where the entire string has to be pulled back every 10 minutes to clear the drilling debris. 

Incidentally, cooling and blowing out is NOT carried out with air but with inert gas - e.g. liquid CO2. 

Since several cross connections between the two vertical deep boreholes have to be created for the "closed loop system" and provided with pressure-tight walls, it would be interesting to know how you would clear a horizontal, 0.5-1km long borehole at a depth of 15km with a bucket! And for several 100 MW, you need - a whole bundle -  such cross pipes as heat exchangers - which also have to be tight for the 300 bar pressure of the supercritical water! Approx. 900 l/s (0.9688 kWh/kg )  for 1GW output power of the generators  - - -  which needs 3 MW of power for the approx. 15 high pressure pumps - to produce that stream! 

e.g. https://de.starpumpalliance.com/pumpen/verdraengerpumpen/plungerpumpen/

Due to the fact  that must be drilled in Granite -  THIS  rock does NOT flow  into the borehole .  
And if  Granite will be vaporated,  than any fluid   anyway   -  that would be a coocking kettle .

And if  the rock will be vaporated - the wall  is melting  and will get a thick and stabile rock glaze !
That stabilisizes and seals  same time like an intruded tube!

Yes: conventional drilling requires much less energy - but takes 30 times longer!

To ensure stability - the two vertical boreholes and the bundle of connecting boreholes of the "heat exchanger" for the closed loop system must be installed in solid granite! 
 -  -  Advance there: conventional 1 to 1.5 m/h - with evaporation 35 to 40 m/h.

Note additional : with increasing depth, the temperature increases by approx. 40k per km.
At 12 km it is already almost 500°C - plus the heat for the drilling process 150-200°C.

The heat for the drill head is then almost 700°C - ! No material can withstand that for a long time.

And: even the best diamond drill head wears out within a few days - then the entire string has to be completely pulled out EVERY TIME to change the bit.

Drilling are NOT be done with air, but with inert gas - e.g. liquid CO2 - in the heat on site it gasifies and thus increases its volume by almost 600 times (under normal pressure) and there is also the enormous increase in volume of the vaporized granite.
This means that the gasified rock does not have time to attach itself to the wall when it condenses until it has cooled down to simple dust!

Bringing the microwave power to the drill head is certainly a problem. The attenuation is actually enormous, especially since there is currently no concept for a FLEXIBLE wave guide several kilometers long.

An existing slim version of the gyrotron will probably have to be modified in such a way that it can be dragged a few 10 meters behind the drill head (heat protection).

Then all that is needed is the inert gas and the power cable to achieve this length - - - intermediate inert gas pumping stations can also be supplied with power via the cable. Since the diameter will be greater than 15 inches for an appropriate flow rate of supercritical water, there is sufficient cross-section for pumps and cables - as well as return flow.

There are certainly still problems to be solved - - and therefore the world's specialized scientific institutions should definitely focus on these problems with the highest priority and appropriate financial resources!
If it weren't for the enormous volume of orders with the insane profit of several hundred billion when converting the economy to wind & PV with the subsequent ruin and the even more profitable reconstruction of the economy - !

Because drilling into natural grown granite (stock) it is almost impossible to come across a gas or oil deposit - and since  THIS non-contact drilling (no vibrations !)  allows continuous sonar measurement, cavities and faults in the rock can be largely avoided!