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