Surely the approach is either protected or it isn't ?
As I think somebody wrote already, once you get handed from approach to tower (which happens when you call Localiser Established) you lose the "radar service".
So the last 6-8nm, possibly more depending on vectoring etc, is flown without a radar service.
My guess is that the tower controller is not allowed to pass traffic info even if can see a radar screen (for the same legal reasons that e.g. London Information cannot), but in an obviously dire scenario the approach controller could tell the tower controller to warn the pilot.
But if e.g. the SE aircraft was not transponding, there would not be a height readout and what assumption would a (any) Radar controller make then? I don't know the rules. I do know that if you fly through the Luton-Stansted gap without a transponder, Radar will assume you are OCAS i.e. below 2500ft (nothing else they can realistically do).
And if the SE aircraft had a poor/nonexistent primary return (e.g. largely non-metallic) then you have nothing at all. But is a zero return possible this close to the radar?? The engine is still metallic, and pretty rough shaped.
OCAS, any "protection" on an approach is no more than what you get when you get under any radar service. They will pass traffic, workload permitting. If the whole approach was in CAS, you get the additional protection that conflicting traffic is less likely to be there because if they were in there they would be illegal (but obviously they can still be there of course). Also different rules apply in that IF the CAS extends down to ground then ATC can take immediate action if they see even a primary return. AIUI.
There are no absolutes.