So what your suggesting is BA, VS and BD have to give up slots at LHR, lets say 10% of their slots each, grounding aircraft and laying off staff in the process, so that DL, NW and CO (two of which are bankrupt) can come in in their place. Do you really think the EU are going to sign up to that?
On the matter of slots simply 'being found', there is an interesting chart
here detailing slot demand and slot utilisation at LHR. Put simply, the slots are highly unlikely to just 'be found' Any latent capacity would have been exploited a long time ago. There are plenty of minnow airlines operating into LHR who could sell their slots, thats where the newcomers will have to find their slots. So far all the USA are offering is unrestricted point to point access between the EU and the USA (no benefit for UK operators) and some loose commitment to easing foreign ownership rules which Congress will probably block. There's not much 'give' there really, is there.