Our IS350 is very much similar with the IS250 in terms of transmission removal. After scratching my head and looking through your write-up, I ended up removing the oil pan to access the torque converter bolts. Those 6 bolts were easily accessible and broken loose with the EZRED 12/14 Extra Long Flexing Head ratchet wrench with a lip. Then using a closed end wrench, I backed the bolts out. This took 30 minutes.

With the replaced transmission, I used a cutting wheel to cut out a section of the transmission where the weep hole is located. This made the reinstall a painless experience. I then cutout a plastic sheet and permatex to cover the transmission portion where I cut.

 

I have an 08 IS350 RWD and followed these guides to remove my transmission. You must remove the driver's side header; you can disconnect the steering linkage and still have enough space. Three bolts can be removed up top, with the rest on the bottom for the header. The exhaust can stay mounted, as can the passenger header. I have a RWD model; this could be different with other models; I'm not sure. Also the transmission cooler. Just unbolt the three bolts and remove the top hose once the transmission is out. To disconnect the halfshaft, I just had to unbolt six 17-mm bolts to disconnect the transmission from the Guido/halfshaft. You can leave the half shaft mounted under the car.