I was having Marshmallow stock ROM rooted with TWRP installed. The way I had installed TWRP recoery made the monthly security updates uninstallable.
After learning the mistake, I decided to download the MOB30D build and flashed as I said in my answer.
After installing the phone was working normally. I tried to install TWRP through ADB command line and there was no error messages. When I rebooted the phone in recovery mode I find a red triangle. After some fiddling with power and volume buttons I saw the following recovery screen and used it for rebooting only:
I tried to flash the latest TWRP version, downloading again thinking the download might be corrupted. I then tried several old version of TWRP and same result. I tried flashing stock recovery, doing all these again with erasing the recovery partition first - all results in red triangle.
I then tried Nexus RootKit flashing stock and TWRP image (downloading again); still no use. I tried Nexus Rootkit's "Boot into temp recovery image" which did not work well but suddenly worked. I flashed latest TWRP image through the temp TWRP and it too says all okay, but no joy:
Question: I am not sure what else I can do to install TWRP. I want TWRP basically to root and for emergency purposes. What I can do further to install TWRP and if not at least root without a recovery partition.
Update I tried CWM recovery as well and same result. I tried to lock the bootloader and unlock again there by resetting everything. Still no use.