My thought on the cracked coil would be that if it is cracked you
should see water leaking. And if it is cracked, then you will most
likely be getting boiler hot water into your domestic hot water. Is
your hot water brown or dirty looking?
As for the pressure relief valve. Check your automatic fill valve. It
may need to be replaced. Depending on your water, the valve may be
stuck open a liitle causing your boiler to fill and raise the pressure.
I'd replace it or better yet, open it up and look to see if there is a
lot of crud in it.

If the boiler is 50 + years old you still may want to consider
replacing it. I replace them, depending of the BTU's and # of zones,
for around 3 - 4K complete. Your savings should be made up in a couple
of years because of the effiency.

A new 40 to 50 gal indirect DHW can run about 600 - 1K depending on
model.

Paul