Thank you for providing that. I have had a read through. Can you telling me if there is any problems with the below plan?
1) Factory reset the tab so it is relatively clean
good, as long as you have the data backed up.
2) use odin to install TWRP (I don't need to root as I am using odin)
good and most probably the safest choice you would make.
3) use TWRP to backup the factory ICS (data, efs, system)
the instinct process we all would do, so that we have a backup to restore in case anything goes wrong.
* will TWRP backup also backup the ICS bootloader? *
yes and no. twrp will backup system data, efs, and boot. the rest of the partition will be the same.
4) use odin to install the JB bootloader
yes, you are right here as well.
* at this point I could contine to run stock ICS but with the JB bootloader?? *
I don't remember exactly, it might or might not cause some issues. I am not sure. some reported they had graphical issues.
5) I can now install a custom JB, KK, LP or M ROM?
exactly. you can, after following the OP instructions of course.
remember it is not advisable to downgrade the bootloader once you have upgraded it to JB, you shouldn't go back to ICS. still if you would like to go stock in future you can download the stock JB firmware from sammobile and flash it via odin.
there is also one issue that keeps me going back to stock ever again. the emmc trim bug that could harm your device.