[Q] Unlock after rooting - is that possible?

So, my friend just bought a Galaxy S6 from T-Mo. Nice device and all.
But... she only speaks a bit of english. Her major language is portuguese, so I installed a custom ROM to enable full portuguese translation. Unfortunately, she called me today about her SIM card.
Her device, right now, is rooted and tied to T-Mo. As the operator states in its website...

Quote:

Device not recognized by your service provider. Please call customer care. Rooting your device will disable the unlock application Download the latest T-Mobile software to continue

Also, from the Play Store description...

Quote:

Device Unlock is an application that allows you to request and apply a Mobile Device Unlock directly from the device. The application is compatible only with Samsung Avant devices at this time.

(she just realized that the phone was locked after she bought the nano SIM, which took 72~96h after the ROM flash, last tuesday, so no way to detect it)
So, four questions:
1) Am I able to unroot and retry the operation? (using, of course, T-mo stock ROM)
2) Does KNOX Status interfere something on it? and...
3) There is a way to request the unlock code from T-Mo?
4) According to Play Store comments, no one succeded in unlocking the device using this "tool". Even Samsung Avant's owners. Is that confirmed?

Thanks in advance. :)

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