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That is a Spring M8 (from the M8_WHL and OP6B700 numbers), so I think that following the 'manual update' instructions here will give you stock software for that device. You will still be S-Off and have the 'super cid', but those won't affect functionality (and if you want to really return to stock state and reset them too then it is much safer to put the rest in a well-defined stock state first).
This process will wipe the phone, so back up anything important first.
Note that being S-Off and having that CIDnum means that the phone would let you flash any HTC software. However, only software for the Spring M8 will work with that hardware, so it's important not to try to use those abilities to flash something incompatible (the 'super cidnum' you have is more useful for the more common M8_UL model, where there are different regional/carrier ROMs that are all compatible with the hardware. It's frankly of little if any use with a Sprint model, as only Sprint firmware is safe to use with the hardware).
Flashing the firmware won't change what bands it supports. This also means that flashing Indian firmware wouldn't give you support for Indian bands, it would just break the phone (because Sprint use different radio hardware).
The Sprint M8 should work on most Indian networks, but to differing degrees. If you visit the site willmyphonework.net you can see how compatible it is with your network.
All that said, I can't guarantee no adverse effect of returning it to stock in your situation (i.e. using it for a different network), because I've no experience of using Indian SIMs in stock Sprint phones (I'm not American, so have never owned a Sprint handset). Why do you want to do this? If the phone is working ok at the moment there may be other ways of solving it.
Following the instructions linked from my first post will return the phone to full stock software, including the recovery (i.e. replacing TWRP). You may have to relock the bootloader to do this (normally running an RUU requires the bootloader to be locked, but as you are S-Off I'm not sure that restriction will apply to you).
That package is called a 'ROM Upgrade Utility' (RUU), but it actually contains a full set of firmware: ROM, recovery, baseband and other bits. And you can't flash it through TWRP: it's a Windows .exe (though there is a way of extracting a package from it which can be flashed using fastboot, but you need Windows to run the RUU part way to do that).
As for the rest, you can relock the bootloader using a computer with 'fastboot' installed on it. Just connect the phone to the computer, boot into the bootloader, select fastboot mode if it doesn't do that automatically, then on the computer use the command 'fastboot oem lock'. I normally test fastboot is communicating with the phone by typing 'fastboot devices' first (should return the phone's serial number), but that's just me.
Returning to S-On is possible, but I have to ask 'why?'. Being S-Off doesn't affect the function of the phone in any way, it just gives you more options, so anything you are unhappy with is not related to being S-Off. You can return to S-On using the fastboot command 'fastboot oem writesecureflag 3', but it's more work to go S-On again afterwards if you change your mind. More importantly, the phone must be 100% stock before you do this, so do not do this unless you have already run the RUU (I don't think you need to reset the cidnum, but you might consider that just to be safe). The reason is that this is turning the software security flag back on: if the phone firmware is not stock then when you turn security back on it will detect this on boot-up and refuse to boot! Now you can flash recovery and ROM when S-On, so they must be OK, but if the phone has been made S-Off that might have been to make other changes and you don't want a brick on your hands. So to be safe I wouldn't touch consider S-On without running the RUU first.
To be honest I've always found HTC rather simple to modify. The security flag (S-On vs S-Off) is the one complication, but you don't need to worry about that if you just want to flash ROMs. As long as you don't buy phones where a carrier has locked-down the bootloader (as Sprint do) it's really easy to unlock an HTC, flash a recovery and a ROM (whereas recent Samsung models are much more effort). That one being a Sprint phone may be where a lot of complication comes from (harder to unlock, not all ROMs will support it).
You can't flash an ruu zip using twrp.
How exactly did you try to flash it from the command prompt? Precisely what commands? You can't just use 'fastboot flash zip file', there's a multi-step process, starting with putting the phone into 'ruu mode'. It's a bit awkward to search up from a phone, but I'll try to dig the procedure out later when I have a computer.