I think it's worth making a few corrections -- showing current loops that go through the inductor is a bit misleading. There are two alternative ways of explaining why this is misleading:
- The two loops are out of phase with each other, so even though both loops emit EMI, they largely cancel in practice
- More simplistically, the current through an inductor is (mostly constant).
For this reason, I believe is more accurate practice to consider the currents in such a system to be a superpostion of:
- A pure AC waveform circulating back and forth through Cin1, U1, D1 loop, plus
- Some irrelevant near-DC flows through LX
Of course, the current flow through LX is not pure DC, but it's typically orders of magnitude below the AC loop, especially in CCM. Now, the ground path from D1 to Cin1 is indeed pretty torturous, I agree with that. However:
I'm not so sure about this, but doesn't a solid ground plane go a long way to cutting down EMI here? The aforemention AC flow can exist as a mirror image in the ground plane below, and this means the EMI is actually going to be fairly minimal? Maybe? Open to correction on this point.
So although rotating Cin1 so that it has a no-via path to D1 (instead of that pointless island of ground) seems like a sensible optimisation, I'm really not convinced that this is a big deal, nor that this 4-layer design would actually perform worse than a sensible single-layer design (not a fair comparison of course, but worth keeping things in perspective nevertheless).