The method is technically more correct than 1, but declaring the key may be a nuisance. Future scale frequencies will be based on this scale until the next change. Each change will be based on the previous key, so that it is possible after several changes to return to the same key but NOT the same frequency. The same as 3, but with a small factor to keep dragging the frequency back to the starting scale. Use the frequency of the note in the scale of itself.
Adjacent keys means keys with one more or less sharps or flats.
It is possible that several instruments of this type could be played together. Then, because they will likely play different note combinations they may not remain in perfect tune with each other. This could be overcome by connecting them together and combining all their notes in doing the chord calculations. This could be done through standard MIDI interfaces which provide all the information necessary. Although MIDI provides all the necessary information about note on and note off, it would be necessary to also have a command to prevent the instruments playing notes that were intended for calculation purposes only.