Hey Nick! That’s right, combined dynamics require the two dynamics to be different (i.e. a variation of p for one, a variation of f for the other).
I can’t remember off the top of my head whether the various dynamic glyphs are available this way, but I would guess this might be simpler to mock-up in Dorico 4 with the Insert Music Symbol/Text dialog, as you can add symbols and glyphs to text items much more easily.
No I don’t think so – in Engraving Options > Dynamics > Appearance you can choose between hyphen, colon, slash, and space for the separator, but there isn’t e.g. a free text field to enter anything else.