Is there a way to add it to the Constitution without going through the legislature?
No. Public initiatives to amend the IL constitution can only affect the legislative article of constitution, and then it must affect both structural and procedural aspects of the legislature.
Are there court decisions interpreting Article XIV.3 that way? (i.e. requiring an amendment to affecting
both structural and procedural aspects.)
I would interpret a
limit to mean that an initiative could not affect matters that are not procedural or structural.
That senators and representatives are elected from districts is structural. That representative districts are nested inside senatorial districts is structural. The manner in which senatorial districts are delineated is structural.
What XIV.3 is preventing an initiative from affecting the content of legislation. Article IV in broad terms describes how the legislature is constituted or structured (Sections 1-4) and how it legislates or procedures (Section 5-14).
I doubt that removing matters from legislative authority, such as congressional redistricting, and vesting it in a separate body would qualify as either a structural or procedural change.
In addition, the power of congressional districting is vested in the legislature by the US Constitution, and the Illinois Constitution does not make the broad claims of the People being the ultimate source of legislative authority that the constitutions of Florida, Arizona, and California do.