To add a second O ring to seal the throttle shaft reliably, ( provided you can disassemble and reassemble the throttle body without damaging the physical and functional properties,) Is to notch the throttle plate at the operating end of the shaft, with the notch slightly shorter than the thickness of an O ring plus a precision ground washer with the same OD with the ID just enough larger to fit on the throttle shaft without any slop, to fit into the notch of the throttle plate. On the free end of the throttle shaft, a ball bearing and a spring are added under the press in plug to press the shaft toward the operating end to compress the O ring for the seal between the throttle shaft and the TB bore.
To remove the throttle plate screws, the end of the screws needs to have the punch staking ground away first, so that the screws will back out without any interference in the threads from the staking. For exact realignment of the throttle plate on the shaft, a pair of small taper pins should be fitted through the plate and shaft before removing the screws. A small arbor press and support blocks or a custom support block, will allow pressing apart and back together the pressed parts.
I will draw up a sketch of the details described above, if you are interested in making a serious effort to add the second O ring seal.
Clearly, using post TB WM injection, preferably one nozzle at each intake port runner inlet, is an easier and a quicker fix. Since WM is best used as an anti-detonation measure, and not as an increasing charge air density method, (although this still happens regardless.) the effectiveness of moving the nozzle down stream of the TB does not reduce overall performance as a practical matter. because the WM volume injected displaces the same amount of charge air/oxygen the maximum amount of WM on a ratio basis of the fuel flow, should be limited to only enough to prevent knock onset and detonation with the most ignition advance and boost pressure combination practical for operating conditions with your setup and tuning.