Late to this thread, but I think Lackey is on the roster for the reasons stated. He's there as insurance in case Arrieta can't start in Game 4 (which is where I think they'll slot him). And he's there out of respect for his career or whatever. If Almora is able to go, then the decision for 25th man basically comes down to a pinch runner, 3rd catcher, or 12th pitcher.
I don't see Martin's baserunning having that much importance. He's not even as good a baserunner/stealer as Quintin Berry who didn't make the playoff roster in 2015. OF defense isn't an issue with Almora, Jay, and Heyward able to be out there late in games. Happ's been good out there. And Zobrist won't kill you in LF.
Rivera has an interesting argument though. Taking a 3rd catcher could make Avila a primary LH PH late in games. He's a really good defensive catcher that can also throw out basestealers. Part of me wonders if they take him at the expense of Wilson or even LaStella. I'd say no for now with the Nationals LH bats and hard throwing RHPs. Those other 2 are shoe-ins to play big roles.
Also, I think Maddon tries to minimize strikeouts while also getting LH bats in there vs. Scherzer. I'd definitely sit Javy if Max is able to go Game 1. But I'd think if that happens you'd see the combo of Zobrist, Jay, LaStella in LF, CF and 2B respectively. Zobrist's obviously has a ton of playoff experience and success, so I don't think you see him sitting in the playoffs, maybe ever. He also has 3 HRs vs. Scherzer in his career.
Of course, you could use the same strategy vs. Strasburg, though he doesn't have any noticeable splits vs. LHBs. He did strike out 13 Cubs in 7 innings in his start this year. So, you could see an emphasis put on putting the ball in play as much as possible.