CHIRP

If the radio can do both split and offset, then we show them both, and configure as they ask

If the radio only supports offset, we expose split, but if they select it for something, we pop up a box that says:

"Your radio can't do this, but it's within range and I can calculate an offset to match, mkay?"

If the radio supports only txfreq, it's more complicated:
3a if it's within the ham band, we let them select offset and hide it under the covers
3b if it's outside the ham band, we pop up and tell them that the radio only has a txfreq, so we're converting their big offset to a txfreq