My 22 rimfire HTA Guardian alum can has carbon locked onto the 1/2 x 20 to 1/2 x 28 adapter for my S&W 22 compact.
What is the safest way to remove this from the adapter?

If you can remove the original 1/2-28 mount from the rear of the tube you can work on the carbon on the interior side that is likely where the adapter is bonded to the direct thread mount. That mount is likely aluminum so aluminum safe chemistry is best bet.

Soak in breakfree suppressor cleaner?

Thanks for the info @KalmanPhilter. Any recommendations on aluminum safe chemicals?

You can try penetrating solvent like PB Blaster, or Kroil, or a 50/50 mix of acetone and automatic transmission fluid and a brush.  But if that doesn’t do it the Breakthrough mentioned above is aluminum safe. There’s an entire thread on it in this sub-forum.

Things to avoid that will attack aluminum are CLR and the dip (50/50 vinegar and hydrogen peroxide). These are both acid based.  Caustic cleaners like original Simple Green also react with aluminum (etching, pitting).  

I suspect your thread adapter extends into the blast chamber past the original threads and build up there is locking the thread adapter in.

Thank you sir! I will give those options a try.