For all who are waiting for results: i got the Huawei E3331 modem working right now. I am still at the internal 3.3V sensor port with the power source. So what did the hack? I ordered a new step-up regulator to power the device. the new one is called U3V12F5 and has a high power transistor on board. With this there is enough current to power high consumption usb stick without any problems. Measurements are about 12x6x3mm and with this it is possible to hide everything under the backcover including the Huawei modem. The Lenovo is powering up without any problems after this there is the usb modem found by windows and you can connect to the service provider without any problems. I made a test to surf around 10 minutes only with the modem activated and everything getting really smooth. So tomorrow there should be a new device arriving here. I will make some hardware tests and if all is good i will do another disassemble/teardown video from this.
Additionally I plan to make a video coverage of the Lenovo miix 2-8 with working modem too but this will take some days. stay tuned.
If there is someone out there who is interested in my miix maybe with the wires still connected to get usb devices working internal please feel free to contact me. For those who dont want wires I can dismantle everything too to bring it back to stock device.