Interesting theory, but I've heard the Flash doesn't actually punch people with his hand when he uses the infinity mass punch. Sounds a little fishy, but I never looked it up.
But one thing is true. It's consistantly shown in comics that when Flash runs into something, he can hurt himself. (broken nose, etc.)
He transfers the energy of the speed-force and the reality that's folded around him to his opponent, which is done before his hand even lands because it forms a barrier around him.
I'd don't recall the issue, but Flash actually ran UP soundwaves like a staircase before to simulate a form of flying. Though he's not used the ability more than the once that I know of.....then again, he rarely needs to.
And in either case, he could enter the speedforce and the force of the thunderclap would be useless, even besides the standing still thing..
I've read the comic that he first used the punch in, and it didn't explain how speed force energy is used in the mechanics of the punch.
In the Hulk's defense, the punch would cleary have a limited effect on the Hulk, considering that it didn't badly injure the White Martian Flash first used the punch on. The Infinite Mass Punch isn't a kill-all.
Alright, so he delivers a couple of these punches to increase the damage he would normally deliver with his 10lb human arm. A thunderclap should still be able to take him out, ESPECIALLY if he tries to phase through it. All the particles that make up the air are moved back and forth with vibrations which could cause the flash to simply fall apart.
Besides the thunderclap, what about hulk striking the ground so hard that crap flies in all directions? Flash IS made of flesh and blood right?
He DID explain it when he punched the White Martian, you might want to read it again. He also did a couple of times in his series. And ... it sent the Martian flying through space...
lol.
But Flash (at least at his normal levels) doesn't just warp in and out of the speed force. He has to accelerate to light speed. Flash ususally doesn't travel at lightspeed either. He often cruises at high mach speeds.
Flash can do Superman/Hulk level punches with the infinite mass punch, it's an EXTREME amount of energy.
And when inside the speedforce fully, the sound vibrations/force of a thunderclap is a million times too slow for him. Stomping the ground wouldn't help much either, since the fight would likely be over before he could even lift his leg, if Flash is to optimum capacity.
Flash CAN hop in and out of the speedforce, and can bring anything to the battle that is under his power as per forum rules [in which Flash is specifically mentioned I might add].. it is still NORMAL Flash, not powered up in any manner. But I do agree, he doesn't usually always travel at lightspeed, he doesn't always have a need to.