The Granita board should already work down to 10 MHz. If you need to go lower, I (or someone else) would need to design a custom RFNM daughterboard for low frequencies. You could do direct conversion up to 150 MHz, so it should be a simple task and come with good performances.

All existing SDR software should be supported via OsmoSDR.

Yes, it should work as a coherent receiver. There are some questions about unmatched I/Q pair trace lengths, but from what I heard from the experts as long as the clock is coherent (which it is), we should be able to correct those in software.