I'm trying to disable SIP to install Sierra.
You don't have to disable SIP to upgrade to Sierra.
If you insist though, try rewriting the Recovery Partition. https://forums.macrumors.com/threads/how-to-update-the-recovery-partition.2010607/
Even on the "unsupported" MacBook Pro 5,5?
Have you ever been able to boot into the Recovery drive in El Capitan previously?
Try downloading the full El Capitan installer app (Install OS X El Capitan) and let it run to install a fresh copy of El Capitan in the internal drive and rebuild the Recovery partition. Hopefully that will get you a working Recovery drive.
Yes, if your referring to a bootable installer USB flash drive.Would it work to boot through a USB drive?
Yes, if your referring to a bootable installer USB flash drive.
EDIT: It might be useful to run Disk Utility>First Aid to see if there are any drive errors also.