Metol is supposed to degrade quite quickly at temperatures above
125F (52C). I don't think I'd worry about a few degrees, but it
is a good idea to use a thermometer.

Normally when you mix developers with metol and sulphite, you have
to add most of the sulphite after the metol. Sulphite acts as
a preservative, but it also prevents metol from dissolving.
Kodak powdered developers appear to have additives which get
around this problem so that all the ingredients can be added at once.
The large amount of sulphite may make the high temperature less
of a problem.

Note than I am trying to answer questions as a learning exercise
for myself. I sure hope Richard or someone corrects me if I'm
way off.

Peter.

 The cooler the better. Use distilled water. 53C is not a magic
temperature. Most formulas mention 125F or 53C when lower temperatures,
even room temperature would do.
 I think some use D-76 at no dilution. It's a thin brew as mixed and
can be used undiluted. Contrast D-76 with HC-110 or Rodinal or some
others where dilutions of 1:25, 1:50, 1:100 and even higher are
used; D-76 1:1 or 1:2.
 D-76 was compounded to be used as an undiluted replenished developer;
that is, to be used time and time again.
 Do you use it one-shot or do you replenish?                      Dan