That scope has +/-25 MOA of adjustment. If your 20 MOA base won't let you zero at 100 yds, then either your rifle barrel is installed crooked by at least 8 MOA, or barrel vibration is throwing your zero off of boresight, or both. Both are common enough that I would investigate them before sending the scope back. Crooked base installation is usually not the problem. Are you shooting a factory production rifle or a custom rifle
Regarding the POA shifting diagonally, it could be due to a windage offset. Is your windage near the mechanical zero when you try to zero elevation, or is the windage off as well?
I saw your post on the hide. IMHO, Sightron does not make high contrast scopes. The high mag models will have low contrast at high magnification. That is normal for Sightron. You will have to move up to a much better scope to operate at 35-50X with good contrast.