Well, you are still going to have to deal with the Harmonizer issue whether you went BK-7m or BK-5. I am not sure a splitter would be needed... I think the BK-7m (and probably the BK-5) have a soft 'Through' for Midi... Just run the harmonizer downstream (or doesn't the Harmonizer have a 'through'?).
As long as you are comfortable playing ALL sounds from a piano keybed, then probably the BK-7m makes setting up and controlling it from an external keyboard the easiest (I don't think the BK-5 has the 'Wizard' setup tools the BK-7m has) but in all probability, the BK-5 can do the same setup, you probably have to set the MIDI up more conventionally.
But the BK-5 has no FC-7 multi-switch input, which means you will HAVE to run much of it from the BK-5's panel.
Decisions, decisions!
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An arranger is just a tool. What matters is what you build with it..!