I highly doubt that 35OrLess has their own roaming agreements. To do that, they would have to have their own carrier ID, like Lyca.
More likely they just checked a box on the TMobile MVNO...
Their native feature probably counts against your tethering quota whereas you may be able to disguise it with mywi.
Yup, it's unlocked GSM-side. Their CDMA phones were also unlocked (SPC -- Service Provider Code -- is 000000), it's just US CDMA carriers will not activate each other's phones by policy.
Install Wugs Nexus Toolkit and have it update the latest unbranded firmware automatically.
Pretty costly repeater. I would be better off to wait till MTS deploys 700mhz LTE, and then just buy a LTE repeater for quarter of that price.