Sorry, but everything you're proposing is not within reach of the current xbox 360 hacking community. I'm guessing it never will be either. Everything you're wanting to do is just competely not feasible through any means. All the pinouts for the xbox 360 are specific, the xbox 360 EXPECTS a dvd drive on the SATA port for the dvd drive, it also EXPECTS an HDD on the custom SATA port for the HDD. Though they both are SATA ports, it's the logic of the xbox one at work here(sort of), if you switched the HDD and the DVD drive on the IDE cable, it wouldn't work(HDD is device 2... DVD drive device 1), because it has predetermined settings of WHERE these devices will be on the motherboard and/or cable. The xbox 360 will not look on another bus/port for the dvd drive, and if you managed to switch them for the correct SATA pinouts of the motherboard to the SATA cable to the DVD + HDD, they would not work, the xbox 360 wouldn't be sending the correct commands to the drives, it would be trying to find the HDD on the DVD drive and the DVD drive on the HDD.
And to the USB dvd drive... that would in essence work, but someone would need to create another firmware for the drive JUST for that, and that won't happen, because there is no reason for someone to need a USB regular DVD drive.
If you're going to mod, you need to void your warranty, that's the price you have to pay for wanting to make more use out of the xbox 360.Here's
a link to a tutorial for having a 120gb USB drive. Well it's not really a mod persay, but it's something.