[HD2 General] T-Macgnolia:
I am still learning about all of this myself and someone else here may be able to give you a more in depth answer. But I will share what I do know with you.

To find if you have a T-Mobile T8585 just look at your HD2. If it has T-Mobile at the top of the front of the phone and it has a green send button and a red end button then it is a T8585. If it does not have this it is a International/Eroupean model. I am not 100% sure about this but you can either go to Settings>About Phone>Hardware information to get the model number or look under your battery. As I said I am not 100 % sure about that as I am on my HD2 right now and can not pull the battery and I am runningvAndroid and not Windows Mobile

As for your limitations question the T8585 better known as the TMOUS has more ROM and RAM than the regular HD2s. Therefore it has a little different internals then regular HD2s. The ROMs made for the TMOUS are made to to take advantage of it's extra ROM and RAM. Just as the SPL 2.08.0000 and SPL 3.03.0000 were made to work with it also. (It is also a SPL 3.10.0000 out for the TMOUS now) Being that the HSPL versions are made from the SPL versions then this is why you only want to flash HSPL 2.08 or HSPL 3.03 to the TMOUS. If you don't you may uncounted problems as these vesionscare made to work with the ROMs for the TMOUS. Now this does not mean that the ROMs for the TMOUS can not be flashed to a regular HD2 it just means you do not want to flash a ROM for the international HD2s to the TMOUS. For one they are not made for it's ROM and RAM, two they have radio versions that contain 51's in them and they will brick a TMOUS. Once you have flashed HSPL to your TMOUS you can flash any custom ROM that is based of of a stock TMOUS ROM but Nome other because like I said if it was not meant for the TMOUS itcwill not operate correctly on the TMOUS

The only compatibility board is the maim board/mother board of the HD2. But I think you can not put a TMOUS main board in a international HD2, bit I do think you can put a international main board in a TMOUS. My question is though why would you want to? Lol

You are correct when you brick a HD2 you have screwed up the IPL hence a brick HD2 will not even turn on. I am not exactly sure why flashing the wrong radio will brick a TMOUS but it THECIPL and the radio must have some kind of direst corolation to each other. Yes when you flash a ROM that has a different SPL version included in the image than it chang the SPL. Unless you have HSPL, but this only hold true when you flash via RUU not via SD card. When you flash via SD card the Bootloader is what actually does the flash and it is only programed to flash the intiror image not just the ROM and Radio if HSPL is present.

A full ROM image contains a SPL version, a splash screen, a radio version, and a ROM version. You can flash any of of these seperate from one another via RUU or SD card but it has to just be the item you want to flash in the .nbh file.

You are spot on with your understanding of whthe the system version, the mantilla version, and the radio version is. I am still not quit sure what the protocol version is myselfcso if you find out let me know.

Yes the same radios that you use in Windows Mobile are used for Android on THE HD2 as radio software is device spacific and can only be used on the device it was made for.