oh there seems to be a lot of confusion! let me clear up a few things.
first, the definitions:
SIM Unlock: allows you to use any SIM in your phone, i.e. the phone recognizes and accepts the SIM without prompting you for an unlock code. it is permanent without special tinkering. no worries with warranty, no need to reverse it.
Security Unlock: this one is what you need for flashing whitestone radios or the patched sprint radio! this unlocks the entire radio and you can do with it whatever you wish.
it is permanent, to reverse it for warranty issues, you have to send a special command to the radio, here is a file to do this for you:
http://hpcmonex.net/htc/secrelock.zip (run it on the phone)
CID Unlock/True SuperCID in Radio: it clears the CID in the radio (note, HardSPL also CID unlocks you in a way, but it's only fake CID clearing) this used to be interesting in the past, on GSM devices not so interesting now. on previous CDMA devices it was very useful to be SuperCID but let's not talk about that.
I don't know if it is still good for this model, didn't look into it yet...
also permanent unless you set the CID to something else, again using special commands.
CID Unlock VS HardSPL: if you just need to flash cooked ROMs, HardSPL will suffice. but note, this package bundles HardSPL too for comfortability.
Sprint Patched Radio: this allows the phone to register on US networks. not permanent, if you reflash radio to an unpatched one, this gets lost.
well, you can flash RhodiumW radios without the SIM/Sec/CID unlocker.
to flash radios from another device, need to be Sec unlocked, but of course it's all in one bundle
it's less than 20 really, see the spot rate on USD/GBP
if you just need to flash roms, then that's already a different package.
in mtty, "set 16 0", or flash a small and neat splash screen using CustomRUU so you can avoid hard resetting.
1. if you just reflash the ROM (OS), the radio stays untouched. all the cooked ROMs the cooks put out are OS-only. radios are usually released in a separate package, and we will be patching some other radios too as we go.
2. on the same phone as many times as you wish
but no need, it's one-use really. the phone won't forget the SIM or Sec unlock that easily
3. yes, secrelocker link is in this post a bit above
you can flash another radio, you'll just lose the US network registering ability. SIM unlock is not the same thing technically.