I can't really see how they'd get their heads together on a deal in which Zimmerman takes a substantial hit this year in exchange for more money down the line. The guy can still hit, sure, but 35-year-old righthanded 1B coming off an .800 and change OPS season with power, walks, and not much else is Mark Reynolds. He's on his second consecutive minor league contract.
If Zimmerman hits substantially better than that, you fall into that grey area where you don't exercise the option but still try to keep him. The problem is that you don't get to that point until at least July of this season, when the tax benefits to this year are substantially less.
The way the Nats likely propose this to Zimmerman is that for nostalgia's sake, they want to extend him for two or three(yuck) more years to keep him a Nat for life and he can platoon at 1B. By stretching out this year's salary and tacking on a few more dollars, it makes Zim look good as giving some money back so the team can extend itself financially for more signings, and it also keeps him in a Nats uniform for his entire career, just like he wants.
Again, I honestly think ownership is seriously considering Kimbrel and also want to extend Rendon and likely needs financial wiggle room to do it.
Between the two places where contracts and AAV are published in a way that it's easy to see are Roster Resource and Cot's. RR has 6.5M earmarked for non-arb players and doesn't have the deferral factor for Scherzer's contract factored in. That puts them roughly 6 Million over what Cot's has, and Cot's doesn't factor in Strasburg or Corbin's deferral factors (neither does RR), so it's quite likely that the Nats are looking at something like 15M currently below the threshold.
An extension that restructures Zimmerman's deal to save even 4M a season and a trade of MAT makes signing Kimbrel possible, but any Rendon extension puts the Nats over the threshold, but the amount over they go is probably what they try to manage as much as they can.
Here is my assumption, any Kimbrel deal and/or Rendon extension will come with deferrals. That should lower the AAV slightly. My guess is 5/90 for Kimbrel , 7/210 for Rendon, and 3/27 for Zim. Say the Indians take all of MAT's salary. If I assume that the Nats get back the "tax refund" for Zimmerman and he only comes in at 9M a season AAV hit, and guess that the AAV for Stras is 23M and Corbin is 21M (assuming deferrals) then the Nats would be over by just over 2M, meaning 1M in tax payed out.