Interesting that you base your premise on the fact that Floyd would come forward on Sergio, because I think that Floyd taking the lead would actually lead to his downfall, in a sense. Even though Floyd has very good accuracy and judge of distance when coming forward, it puts me on edge every time he leads into his punches. Few fighters have ever tried to make Floyd lead, and I think Martinez would try to exploit this by baiting him. I noticed that against southpaws, when he tries to come forward, he sometimes follows them around the ring, and walks into shots he normally wouldn't.
Here, at :30 secs, Mitchell catches Floyd with a few stiff jabs while circling, and Floyd is planted since he's stuck on the front foot.
From 3:40 on, you can see Floyd simply following whenever Mitchell steps. The reason I point this out is because Martinez's favorite tactic is to walk towards the right hand when out of range, so that his opponent follows and cedes the space he needs to circle back the other way and regain the center of the ring. I think Floyd trying to walk him down plays into his hands. 4:09 shows Floyd reach uncharacteristically, and almost/arguably loses the round on ceding the angle.
Here he is reaching against Cotto, lifting his right leg off the ground and stepping forward into a lunge nearly ever time with his chin in the air:
Then of course there's the right-hook counter knockdown against Judah and the handful of counter-lefts he walked into.
I simply don't think Floyd is stylistically suited to come forward against Sergio (as a main strategy anyway, in spots it could work great). At least, I'd feel nervous as a Floyd fan watching him leap into shots.
The point you mention about Barker, however, is a way I think he can get at Martinez. If he sports a high guard but hangs back a bit while engaging, he can anticipate Sergio's jab and knock it down, since like you said, it's already down low. Floyd's parrying skills against Judah show that he can paw and neutralize certain key punches from a southpaw if they stay still trying to play chess with him, including the jab. If he can catch Martinez side-stepping without throwing or rushing forward with check hooks and rights to the body, he can start to dismantle his offense.
In terms of in clinches, I think it's anybody's game. Floyd is defensively superior, but Sergio will no doubt throw in more volume and with much more potency. He did after all break Jr.'s face. I don't think the fight would end up here often except in those award moments of coming together that always happen between opposite-stance fighters, but if it did, I think it could favor Floyd in terms of points within the round, but also Sergio in terms of damage done to Floyd which could affect him down the road and overall in the fight, especially considering Floyd's one nearly always open spot is the right side of his body (hard right hooks).
I think I'd favor the former scenario to occur and pick Sergio, but I think Floyd can win using elements of what you said and what I added. Good post.