You seem to be missing my point
"Nonetheless an inital determination must be made by the engager even before offering someone a contract. An IR35 tribunal would take actual working practices in to account, but no contractor is going to accept a contract on the basis of "we'll make an IR35 determination after you've signed the paperwork"
Until April 2020 the determination as to whether a role is or is not inside IR35 is up to the contractor to make the engager has no say in it AT ALL. It is currently 100% up to the contractor to do their due diligence, get the contract reviewed and if needed amended and then they can make the determination as to whether they are inside or outside IR35.
The client and the agent have no say whatsoever in this process until April 2020 unless the role is in the public sector.
"we'll make an IR35 determination after you've signed the paperwork"
Is utterly nonsensical as they dont make the determination before, during or after you sign the paperwork.
You get offered a contract you get the contract on offer reviewed, you make your initial determination and then a few weeks in to the contract (assuming you take it) you have the working practices reviewed and make your final decision - in some cases you can get the client to help you determine working practices up front but in reality that's rare.