I think ya'll are reading too much into this. If you make the bolt swap - and dispose of the bolt that would make it a semi-automatic - you no longer have a semi automatic "assault weapon". A pistol grip, handguard, threaded barrel, flash hider, collapsible buttstock, etc etc would not make the weapon a semi automatic firearm - only the bolt, which you replaced would.
The bolt is not an "attachment", it's an integral part of the firearm, cannot be operated without one, and in this case it is a single shot, and no longer semi automatic.
Having said that I'm not a lawyer, I have no dog in this fight, and no longer live in Illinois, but if I did, I'd take this information to your own attorney that would be defending you in court and ask them for their instruction. Any opinion on this board in just that - an opinion. Either way best of luck getting this law overturned / thrownout.