Hey bro, I decompiled the stock boot.img to see that if 'verify' flag was preventing booting the system with other than stock recovery. Dm-verity is a recent security control for preventing booting with changed/modified kernel/system. But I could not find any such flags, but found verity_key, so I just deleted it and decompiled the boot.img. I am not sure whether it can solve your booting problem. Let's see will this now allow to boot with custom kernel. Here is the modified stock boot.img. http://www.mediafire.com/file/tc1k1g..._oppo_boot.img
Flash first this boot.img and then flash the custom recovery.imgs (you can try your recoveries also)
I have also ported two recovery.imgs. Both are ported from the twrps for the same device, K6000 plus. However, what I found for this k6000 plus was different in size. So I ported two twrp recovery imgs. Here are two twrp ported recoveries, one is from you have given links to and another is from what I found on this forum.
Once again, I reiterate that first flash the modified boot.img and then try flashing the recoveries.
Best of luck
he used modified preloader.bin to boot into custom recovery.img so i assume that preloader.bin have keys/checksums to verify the boot and recovery images
the same topic was going on xda
but unfortunately i have no enough knowledge about editing preloader.bin so it may take some time google/learn and flash.
i believe your custom images will be prefect after deriving solution. i will update my progress soon. please do share if you know anything about the same.
Thanks in advance