I'm sorry to hear that transferring the 7.5 NAD resources into the 7.5.3 DT didn't work for you. Anything is worth exploring at this point.
The System 7.5 NAD's extra resources are not the same as the respective resources in the full system 7.5. So using the 7.5.3 installer on top of the 7.5.3 disk tools disk is not similar to the method that Apple used to make the 7.5 NAD. It's possible that the 7.5 NAD's extra resources are from an older system or maybe they were even created specifically for the NAD disk. Maybe it was space, maybe something else, but I'm sure they had a good reason.
An alternate approach would be to take the 7.5 NAD and systematically update it towards 7.5.3 by replacing its resources from the 7.5.3 DT a little at a time, trying to keep it bootable and keep the networking going. At some point along the line, it may turn into some sort of PPC-bootable hybrid disk, part 7.5, part 7.5.3.
The System 7.5 NAD's extra resources are not the same as the respective resources in the full system 7.5. So using the 7.5.3 installer on top of the 7.5.3 disk tools disk is not similar to the method that Apple used to make the 7.5 NAD. It's possible that the 7.5 NAD's extra resources are from an older system or maybe they were even created specifically for the NAD disk. Maybe it was space, maybe something else, but I'm sure they had a good reason.
An alternate approach would be to take the 7.5 NAD and systematically update it towards 7.5.3 by replacing its resources from the 7.5.3 DT a little at a time, trying to keep it bootable and keep the networking going. At some point along the line, it may turn into some sort of PPC-bootable hybrid disk, part 7.5, part 7.5.3.