So I can parallel either diodes alone, or rectifier blocks to get
increased current capability? I understand about the cascade failure

You cannot parallel diodes directly because they will have a small
voltage mismatch and one diode will hog all the current. You
can only parallel diodes if you force them to share current
by putting a small resister in series with each diode.


if one should go. Since the charger has about 6 taps on the primary,
for increasing rate of charge (simply higher voltages) can't one just
select a lower setting and ignore the voltage drop the old selenium
rectifier had?

Seems very reasonable and worth a try.