Vas: IWM ("Integrated Woz Machine") is the single-chip Woz controller. It can handle 400k and 800k disks (plus Lisa twiggy drives and Apple II/III 5.25"), all of which are GCR.
SWIM ("Super Woz Integrated Machine") adds MFM capability and can do 720k MFM, 800k GCR, or 1.44M MFM (when connected to what Apple called "SuperDrive" at the time). The registers are largely identical to the IWM (NetBSD is capable of driving the SWIM/SuperDrive in 800k back-compatibility mode only) but it's sufficiently different that just slapping an IWM+800k drive there makes the firmware quite unhappy on SWIM-equipped Macs.
The original Mac II did have an IWM + 800k drive and that might be our way in, but that particular ROM is very hard to find. (Later-model IIs had the SWIM+SuperDrive just like later-model SEs did, and the upgrade uses the same firmware that the IIx, IIcx, and SE/30 did).
eta: that's unrelated. Everything I have shows block 0 as complete and valid (the recently released HFSExplorer, not to be confused with the old HFVExplorer, opens -extract'd CHDs fine). Just the newer firmware doesn't like it for some reason.