FTL is mutually exclusive from the optional modifiers. You aren't looking for FTL here, you're just wanting the optional modifiers.
I guess im not so smart cause I don't fully follow you. With me you gotta draw it with crayon.

I'm trying to learn everything regarding isboxer spawned macros in the context of preventing key conflicts, and learning how to troubleshoot key conflicts.

bottom line as i understand it currently.
1) If i have backspace slaved to a {FTL} based macro with "Can use FTL Modifiers" checked then all eight combinations (backspace, backspace/alt, backspace/shift, backspace/ctrl, backspace shift/alt, backspace/shift/ctrl/, backspace/alt/ctrl, and backspace/alt/ctrl/shift) are blacklisted in the sense that i cant use them to validate further macros. Without being ignored by is boxer or creating a conflict.
2) If any or all check boxes are ticked in "Allow Extra Modifiers" i cannot use those checked modifiers to to build any additional macros using key combinations used.

example:
nuke
/cast [nomod] nuke
/cast [mod:cntrl] instanuke
with R selected as the base key. I check off ctrl with the "alow extra modifiers" I can still use key combo's R+shift and or R+alt