thermostat, heat pad, temp gun, need some validation, OH MY.

[Evenstar]

Cover the screen top completely leaving just an opening big enough for the bulb. This will hold in more heat and humidity.

Lose the bark. It's too dry and it will mold very fast if you try to mist it down for humidity.

The best substrates are newspaper, paper towels, cypress mulch, or Aspen.

Aspen will also mold quickly if misted for humidity but if kept clean and changed this should not be a problem.

*edit*
You can use foil taped down to cover the screen top. You can also get a thin sheet of pink insulation, paint it whatever color you'd like, cut it to fit the sides and back of the tank, and put it on the outside of the tank. This is decorative, will help keep the snake calm as it cuts out reflection and glare, and it adds a lot of extra insulation

You can get creative!

Make sure you have at least one (preferably two) tight fitting hide boxes for the BP.

x2! The bulbs will suck the humidity out really quickly. I know you need the bulb to bump up the ambient temps - I need to as well on my baby boa's tank - so you need to do something to the screen top to help hold in the humidity. I like to use wax paper sealed along the edges with duct or foil tape - it lets in light but closes the top really well.

I recommend cypress mulch or coco fiber for substrate - this will help with the humidity too. And you won't need a thick layer - no more than 1/2 an inch. Too thick and heat can build up and actually make the hotspot too hot. Less is better.

To answer your specific questions:

A) How accurate is the probe from the HELIX unit? IS the foil tape possibly skewing these numbers and should I take a different approach? IS the differential temperature that great between the 3 devices? Who should I trust...
Your temps actually sound pretty decent and as long as they're stable, you should be all set. HELIX are actually pretty darn reliable. The foil tape should not be skewing the reading enough to be worried about. The differential temp between the 3 units aren't an issue. First, they probably aren't that far off from each other and second, absolute precise temperature to the single degree is not necessary as long as the temps are STABLE. If your ambient temp is consistant between 76-80 and your hotspot between 90-93, your snake will be fine.
B) Should I add a 50W Infrared bulb over the middle of the tank to increase ambient temps?
I wouldn't add any other bulbs. The more bulbs, the less humidity and that is more important since your temps are more or less fine.
c) Should I face the temp probe from accurite arched side facing down, facing up or remove the adhesive tape and tape it down right to the tanks bottom over the helix probe of course both probes being on opposing sides of the bottom glass.
The Serpent Merchant answered this for you with his pic. The flat side of the probe faces down.

Hope this helps you! Can't wait to see pics of the little guy!