An important contemporary approach to the two-slit situation you ask about is decoherence.
Many contributors to this forum are fans of decoherence, I am not, but it is very important and worth attention. I couldn't help noticing you still had queries about it even after the explanations of these «fans».
Since one should always be able to include more of the outside world in the box of quantum analysis, always be able to push the boundary out, you are basically asking if any progress has been made by including the slits, and detectors, in the unitary Quantum picture, or if instead any progress has been made by changing the unitary picture even a little. The decoherence approach does not change the unitarity of the evolution, and neither will QFT. (Some have wondered if gravity or other non-linearities will, but we will not go into that right now.)
Brief sketch of decoherence
For simplicity, assume there are only two ammeters behind the slits. As always with unitary evolutions, the electron, after being diffracted by the slits, and when it reaches the plane of the ammeters, is in a superposition of two states belonging to the different locations on that plane of the two ammeters (normally it will be in a superposition of many states but we can simplify here, too). $c_1\psi_1 + c_2\psi_2$. (We will simply neglect the occurrences where neither ammeter fires.) The state space is effectively C$^2$. Now, the ammeters are being modelled quantum-ly, too, so the set of two ammeters has a Hamiltonian and a Hilbert Space for the state vectors (wave functions) of the system of two ammeters, $H_{amm}$. The state space for the combined system of electron--being--measured and measurement apparatus (two ammeters) is then C$^2\otimes H_{amm}$. The decoherence approach makes more or less the same key assumption Mott, London, Wigner, and many others make down to today (but which I query, elsewhere, see my other posts and links), which is that whatever dictionary or correspondence there is between the macroscopic idea of «ammeter 1 fires» or, parallellly, «ammeter 2 fired», is to be modelled by a collection of quantum states in $H_{amm}$ (this is what I usually complain about, but not here), or, since we can pass to their closed span and simplify to assume there is one state, label them $\phi_1$ and $\phi_2$. (A key point in this regard will be expanded upon in a minute...) Now since this means that $\psi_1 \otimes \phi_o$ evolves unitarily to $\psi_1 \otimes \phi_1$ (Here, $\phi_o$ is the initial state of the ammeter-lattice while it waits to discharge or fire) but that $\psi_2 \otimes \phi_o$ would evolve to $\psi_2 \otimes \phi_2$, by linearity (no progress has been made, IMHO, by tinkering with the assumption of linearity), what actually happens is that our electron and ammeter as a combined system evolve to
$$c_1 \psi_1 \otimes \phi_1 + c_2 \psi_2 \otimes \phi_2,$$
which is an entangled state: neither the ammeter nor the electron can be considered as a separate system anymore.
At this point, decoherence, depending on which flavour is administered, points out something undeniable: in fact, there are many more degrees of freedom within the ammeters: our simplification (which is the same as that of Wigner, EPR, and many others) overlooks something important. It is, according to this theory, important that the ammeters are at least weakly coupled to the environment. It is not quite a closed system, so the analysis above is only approximate.
All decoherence approaches (as far as I know) use density matrices. (I would be interested in a current reference to one that works only with pure states and not density matrices.) It can be shown in theory, rigorously, that this coupling with the environment leads to a further thermodynamic-like evolution to a density matrix which is very nearly diagonal and so can be regarded as a classical (or Bayesian) probability distribution on the two states, which are each obviously separable:
$$\psi_1 \otimes \phi_1$$ and $$\psi_2 \otimes \phi_2.$$
The Coleman--Hepp model and Bell's response
In my opinion, the grand-daddy of all decoherence theories is the so-called Coleman--Hepp model.
I learned it in Bell's famous paper,
a freely available copy of it is here: http://www.mast.queensu.ca/~jjohnson/Bellagainst.html
in which he tranlsates it from the language of QFT in C*algebras to the Schroedinger picture and Heisenberg picture. Not on Los Alamos archive. Of course Coleman and Hepp are two most distinguished physicists. Briefly, the criticism, which I agree with, is that a density matrix is not a state, so this is really no better from a logical or foundational point of view than the open system approach and suffers from the same question-begging. (Which is more than thiry years old now, so it's not progress.)
The Physics of these models
These models all use a kind of thermodynamics, and this is surely right, as Peter Morgan (hi peter) said in a previous post, the possible detections are thermodynamic events...and the way to make that precise is to take some sort of limit as the number of degrees of freedom goes to infinity. But none of these models reflects in the model that measurement is a kind of amplification, none of the thermodynamic limits involved uses negative temperature, and this is surely wrong. Feynman's opinion was that this was decisive, see Feynman and Hibbs, Quantum Mechanics and Path Integrals, New York, 1965, p. 22. So these models do not incorporate Feynamn's insight. The models of Balian et al. cited above do incorporate this, and study phase transitions induced by tiny disturbances from an unstable equilibrium which is indeed the physics of bubble chambers and photographic emulsions. Bohr thought that the apparatus had to be classical, and decoherence models do not use a limiting procedure that introduces a classical approximation. So they do not incorporate Bohr's insight.
There is experimental evidence that interaction with the environment induces decoherence, but not yet in a way relevant to measurement. There is also experimental evidence , the so-called spin echoes, that mesoscopic systems can recover their coherence after losing it, which is what Wigner always assumed.
