If the phone is booting up in Download mode, and you're not explicitly pressing the key combination to cause that, then it sounds like you buggered something up pretty badly in your "unlocking and rooting" spree. Once the phone is in Download mode it'll stay there until you actually download something or pull the battery.
Forgetting all of the stuff that got you into this mess, the first thing to do is to make sure your PC has the correct drivers installed to recognize the phone. After the phone boots up in Download mode, THEN you should connect it via USB. If Odin doesn't recognize that a phone is connected, try a different USB port on the computer.
You could try installing the Kies software on your PC - it'll install the correct drivers for the phone (do this without the phone plugged in). If that works, you can actually use the Kies software to flash the correct firmware.
Sent from my rooted, debloated, deodexed Sinclair ZX-80 running CM -0.001 using Tapatalk 2