USB interface while running micro at 2.5V
Is it right the USB will not work correcty running the micro at 2.5V?
  


Per STM32F datasheet, the lowest core VDD to work with USB is 2.7 V, although it is a stretch. The MCU might receive USB packets well, but protocol responses will be too weak for a standard USB host to understand. So, running the MCU at 2.5 V would need some level translation, and not just translation, but it better be something that resembles USB FS PHY drivers.  


  
  Will the TXS0202 work if it's connected to a cabled interface on the high voltage side?
  


The TXS0202 is designed for INTER_CHIP (logic-level) communication (There is some obscure addition to USB for FS mode; there is also HSIC - high-speed inter-chip protocol). The TXS0202 has some integrated pull-ups and pull-downs, so I would be hesitant to use it to drive open USB standard, these pulls might violate standard signal levels/states and confuse standard USB host.


  
  If point 2 is no, can I actually do what I need to? Are there simple devices that will level shift for cabled interfaces?
  


I would consider some general-purpose level shifters, uni-directional, like 74LVC2T45, although no guarantee, and good testing/validation will be required. Maybe some older simple "Q-switches" will work, PI4ULS3V302 from Pericom/Diodes. MAX3372-3379 might be considered. 

Solution to the typical discrepancy between the trend to use low-voltage cores and USB protocol (3.3-V signaling and 5-V tolerant) is not to use the embedded USB PHY, but use an external USB PHY connected to MCU via UTMI+ or ULPI bus. The ULPI PHYs will take care of the level translation, but you might need to use a bigger variant of STM32 processors, with ULPI interface.