[AT&T Samsung Galaxy S II SGH-I777] creepyncrawly:
Yes, questions are usually supposed to go in the Q&A forum, but there is not so much activity in this phone's forum any more, so it really doesn't matter much. And this information is more of a general nature anyway.

1. Hard brick on an android phone generally means that one of the boot loaders is corrupt, or it might mean that the memory module section that contains the boot loaders or other low level code is damaged. In general, the main thing you have to be careful about is when flashing a boot loader to make sure that the flash is not interrupted. For instance, say the power goes out, or the dog pulls out the usb cord, right in the middle of the flash, and after the boot loader partition is wiped, only part of the code is copied back to the partition. The good news is that the individual bootloaders are fairly small, so the time of vulnerability is a matter of seconds.

If you need to update to ICS or JB boot loaders, you would have to flash the full stock distribution that has the boot loaders included. No one has made Odin flashable tars of either of those. The UCKH7 Gingerbread secondary boot loader is available in tar, and that is the only separate tar I know of.

2. I don't know software engineering, only a little programming. I don't know where the code that puts the phone into download mode is located. It seems likely that it is in the secondary boot loader, but that is only speculation. I do know that you can enter download mode, and then flash both boot.bin and/or sbl.bin.

1. The memory is partitioned. Each chunck of code is loaded into its specific partition. I don't have a partition table handy for the S2, but essentially you have: primitive boot loader (boot.bin), secondary boot loader (sbl.bin), parameters (param.lfs), kernel (zImage or boot.img), cache (cache.img), system (factoryfs.img), hidden (hidden.img), modem (modem.img) and several others like PIT, EFS, CSC and I don't remember what. But the ones I named are what is included in a full firmware distribution, and the AT&T model does not allow for the changing of the CSC like on the international S2 so that is not used. I'm not a Linux person, but if my understanding is correct, the img files install like a block device, but the boot loaders and param at a lower level.

2. There may be a recovery partition, but I'm not sure of that. If there is, it isn't used. Anyway, you are correct that the recovery is compiled into the kernel and is installed as a unit on the S2. You can not install a separate recovery on the S2. Many Android phones, maybe most as far as I know, do have a separate partition for the recovery. The S3 and S4 do also.

3. If you are interested, I have attached a partition table for the S4, which you might want to look at just for interest and learning. If memory serves me, it is quite a bit different from the S2.

I would assume that the above is correct. The boot loaders in the 4.1.2 UCMD8 firmware would be the latest official ones for this phone. As far as flashing back to earlier stock, you would only get the boot loaders if you use a full distribution. Many of the stock distributions and almost all of the custom firmware posted on this site for the AT&T S2 do not contain boot loaders or param.lfs.

A lot of people around here (myself included) speak from anecdotal information gathered from the forums.