"3. The installation instructions for the Phase III heater explicitly
specify not to plug the old hot water coil (their exact words:
"Plugging tankless coil inlet and outlet will [sic] result in severe
personal injury, death, or substantial property damage."). Now first,
it seems to me that if the coil is completely purged of water, the only
thing in it will be air, and expanding air won't be enough to rupture
the coil. Additionally, I've seen similar systems in other homes where
the HWC HAD been plugged; also, since our coil is already cracked,
additional damage will be minimal or non-existent. Thoughts? "

If the existing in-boiler hot-water coil resides INSIDE the water in
the boiler, if it leaks it will fill with water. Plugging it in that
case could be bad news.

Leaving it open would mean water would be forced out of it, the rate
depending on the size of the leak.

You could T the 2 ends of the coil together and connect the 3rd leg of
the T to a std pressure relief valve, or you could connect the 3rd side
of the T to one of the boiler-water legs, but it all seems somewhat of
a hack.

If the coil is not submerged in the boiler water, it would seem OK to
plug it (otherwise you'd get fumes through the leak), but I'd seek
professional help considering all the questions involved.

dave