Does the SMiRF take care of all data validation problems so that we only get clean serial data out of it? Can I use 2 remote units together if I don’t need USB? Can I use Screamer to send boot loader programming to my own mcu over the SMiRF wireless connection?
The SMiRF takes care of all the packet validation so that you get a clean signal - Yes.
You can easily use two remotes together - but no more (currently). The base and remote units are actually identical, it just that one has a USB connector. You could use a ‘remote’ attached to your computer by other means.
Boot loading could be done - but you would need to take into account some timing, such as when a packet takes multiple re-transmits, it takes a bit longer than just sending a character down the RS232 port.