Okay so you have
oiX = oX / 2 + Y * -1 * iH + iXo
oiY = oY / 4 + X / 2 * iH + iYo
Multiply by 2 or 4 to get rid of the divisions first
2*oiX = oX - 2*Y * iH + 2*iXo
4*oiY = oY + 2*X * iH + 4*iYo
Plug in the definitions of the origin and iso height and offset
2*oiX = X*TW - 2*Y * TH/2 + 2*iW/2
2*oiX = X*TW - Y * TH + iW
2*oiX = X*128 - Y*128 + 64 (1)
4*oiY = Y*TW + 2*X * TH/2
4*oiY = Y*TW + X * TH
4*oiY = Y*128 + X*128 (2)
If you add equations 1 and 2, the Y cancel and you get
2*oiX + 4*oiY = X*256 + 64
2*oiX + 4*oiY - 64 = X*256
(oiX + 2*oiY - 32)/128 = X
If you subtract equations 1 and 2, the X cancel and you get
2*oiX - 4*oiY = -Y*256 + 64
2*oiX - 4*oiY - 64 = Y*-256
(oiX - 2*oiY - 32)/-128 = Y