The most extreme scenario is Nadal playing and reaching finals at both. Fed would need to make finals both times and beat Nadal. That will give Fed +800 over him. Subtract 710, and Fed would be ahead by a mere 90 points at the end of Miami.
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I don't think Nadal comes close to replicating his clay season, so Fed should take over #1 in May sometime again even if he just sits home and sips glasses of wine.
We could be treated to the bizarre scenario this season of the No. 1 ranking being swapped back and forth, with the guy who is not playing at the time backing into the honor. If Federer has a mediocre IW/Miami combo, even an injured, idle Nadal could assume the lead. And as you point out, Federer could retake the top spot as a spectator if Nadal fails to defend most of last year's clay points.
At least Federer finally grabbed No. 1 by actually winning Rotterdam. It would have been much less satisfying to achieve "oldest No. 1 ever" status simply because his rival kept pulling out of events.