Sorry to hear about the dead Teensies...ouch.
Fascinating problem. After reading you email a few times and to make sure I didn't jump to conclusions:
1. your "spy" connection towards the usb/TTL converter should not have the TX connected at all, only RX and Ground.
2. Could there be a possible ground issue via the power supplies of the PC and the uBitx. Try with the laptop on batteries if that is what you use as a computer.
3. When you say that "And i also installed a new level converter - the level is higher now." can you check with an oscilloscope? That should show you what happens when the power is applied to the uBitx.
4. The schematic of the Arduino Nano shows a 1K resistor between the D0(RXD) and the USB converter chip and the same for the D1(TXD) one, so your TTL level converter should be connected directly to the D0 (and D1) pins on the Nano, not through a resistor.
5. Just re-checking the schematic of the Arduino nano the built-in usb-ttl converter is always powered up, even if the usb cable is disconnected. So that may interfere if you have a (high) resistance between your level converter output and the D0(RXD) pin of the AT-Mega.
6. The normal idle state of serial TTL is high (if I am not mistaken) and the USB chip will pull the RX (D0) high through the 1K resistor, but your level converter should be able to pull it down to low level anyway (or vice versa if it's the other way around).
7. The fact that when you use your external usb/ttl converter with a PC and term emulator you get comms shows that it can be done (if I read your email correctly). What do you use for your Teensy to Arduino level converter?
Scratching my head on this one really..hihi. Let us know what you find.