GiRa

If you use google chrome, close it, reboot the machine (for real!) and then try again directly, without opening chrome. That browser tries to steal midi devices.

I have a Standard, but does the DI-box work? Can you check with a multimeter?

That is not possible, but consider enabling the 12 string "mode" and muting the main string. It can lead to better results sometimes.

You can raise the bridge a little bit, while lowering the saddles, and get more range. Any competent guitar tech can perform a good setup on your guitar. If you take an acoustic model and play the harmonics at the 5th, 7th, and 12th fret, you will be able to identify if the intonation needs adjustment. If the intonation is not correct, you will hear some artifacts under the note.

I suggest you to setup the bridge plate parallel to the body, so the bridge will be very smooth and you will be able to setup the intonation very well. Setting it up non parallel will give you more range when pulling up, but it will also make the setup a little bit harder. A very tight intonation is paramount for good alternate tuning performance.

I had this very same issue on the harmonics (especially with the 12 string acoustics) when my setup wasn't perfect. Especially the intonation.

The standard can get down there. One thing to keep in mind is that, when you pitch shift that much, the feel when playing the guitar will change.

I don't have a Shuriken, but this applies to any Variax so... 1- The Variax just shifts the physical tuning of each string so you have to think about that. All the "Shuriken" preset are mad for a physical drop D tuning. The standard tuning in the Shuriken is actually a standard tuning with the sixth string raised one step. 2- I don't have an Helix and can use a Variax with any other amp, by using the battery. When you use the USB adapter, you have to use the battery. Don't know what happens if you use the Helix as a Variax interface.

I would bend the strings behind the nut or behind the fretting hand.

Piezos are pressure-based transducers, the guitar will receive a stronger signal (thus track better) with more pressure on the saddles. There are other ways to achieve this, for example by having a stronger string angle at the saddle. It is pretty much the same principle of setting up an acoustic guitar with an undersaddle pickup. Moreover, more tension means a tighter swing of the string when vibrating, thus letting the player use lower action with less buzzing. In the Variax world this means less artifacts.

Knowing which note you are playing so the guitar can perform the alternate tuning magic ;)

The longer scale was meant to get better note tracking, not for the "pling".

Maybe the bridge. Most of my guitars are Floyd Rose equipped and a vintage tremolo is a super duper set back for me. A two pivots bridge would be probably better. You can setup the Standard bridge and get a very good performance tho. :)

I own only a Standard, so I can only reply to question 2: they are the same electronics wise.

Do you have any DAW or Google Chrome running? I don't know if the Variax interface is MIDI-based, but those programs could "steal" MIDI devices from other applications.