Superman ALWAYS holds back. Keep that in mind. In the Doomsday fight, he just stopped holding back in the end, and we saw what happened then. A non-holding back Superman is going to shatter Hal's shields. And kryptonite is in no way, shape, or form an auto-win against Superman. His resistance to Kryptonite had been steadily increasing every year to the point where he's trading blows with SBP on a planet of the stuff. And there have been many other examples posted in this thread of him resisting it as well.
Also, if we go by your scenario, where Flash goes to lend GL speed, well, that means he's distracted (even if for a mili-second) and that's enough time for Superman to move in and beat Hal's brains in, or shatter the shields. And MMH's TP is going to get to Flash if Hal isn't shielding him. And you're giving Hal a lot of credit thinking he can simulataneously:
1) Shield them both from TP
2) Resist Superman smashing the shields
3) Fire thousands or kryptonite bullets at Superman
AND:
4) Give Flash a ring.
That's just not going to happen. And really for their team to win, he has to be at least doing 3 of those things at once. WHILE Flash is distracted giving Hal speed. And if he's giving Hal speed, he's definitely not stealing Superman's at the same time.