Any Lorentz transformation is simply a hyperbolic rotation (+boost) of your coordinate system - you are merely labelling the same physical events in your spacetime in a different way. You are always free to do this, since it has no physical consequences in the classical world - the energy-momentum tensor is the conserved Noether current associated with time translation invariance, so you can choose to do this either in the future direction, or into the past, the difference just being a sign convention. The actual dynamics of the system are the exact same, so no laws of physics change form.
So of course you can describe classical anti-particles as propagating backwards in time (relative to their positive-energy counterparts), but that just means you’ve chosen a different sign convention in your description of the system. And again, all Lorentz transformations (antichronous and orthochronous) are diffeomorphisms, so applying them to a given metric does not change anything about the geometry of that spacetime.
Sure. However, this isn’t the only thing it does - it also changes the spatial components of the 4-vector such that its overall norm remains conserved. This is why 4-vectors (more generally: objects that are representations of the Lorentz group) are covariant under all Lorentz transformations, and laws formulated with them retain their form.
There is no change in particle trajectories, such a spacetime has the exact same geodesic structure, only future-directed time-like unit vectors are now sign-inverted, so all processes “run backwards”, and all energies are negative wrt to the ordinary case. All curvature tensors and their invariants remain unaffected, so this is the same geometry described simply in a sign-inverted way.
To make a long story short, the point is just this - if you start off with a positive energy-momentum tensor and run through the maths, you don’t end up with a negative-energy region beyond the horizon during a collapse process - at least not in any solution that I’m aware of, since M is a global property of the entire spacetime.