Yes, instrument changes can happen mid-system, like in the example on the linked page.
Frame notation: this isn’t possible natively yet, but there are things you can do with lines that have long hooks (there have been discussions around this on the forum before) or using text frames with borders, but those are locked to the page rather than the music (they won’t move if the music moves).
Eb clarinet: Dorico has an E flat version in the instrument picker – search “clarinet” and pick the appropriate transposition in the right column.
Hiding noteheads: you could create an invisible notehead, or you could simply adjust note spacing to leave a gap. It depends on what feels better or makes most sense in context. Example for invisible notehead linked here – the relevant dialogs are the Edit Notehead Set dialog and then the individual Edit Notehead dialog.