I have now completed this 12 month keeping test in which ~20ml D-316 concentrate in a 200ml bottle was exposed to air for ~30 min about once a month (1 year sample).
(1) HP5+ was developed in different quite fresh 2 months old D-316 for 2 x Xtol 1+0 time.
(2) HP5+ was developed in the above mentioned 1 year sample of D-316 for 2 x Xtol 1+0 time.

There is little difference between the negatives from (1) and (2) except there is a slight loss of density in the negs from (2), the 1 year sample.

Considering that this is a rather extreme test using only a small quantity of D-316 in relation to the size of the bottle, which was opened many times, and it is unlikely anyone would repeat these extremes in practice, IMO this is a satisfactory result.I believe Mark is running a keeping test which I hope would more closely represent conditions of practical use,and there may be results to come from this later.

Regarding the chemistry of deterioration of D-316,
Ascorbate oxidation is probably the cause of the slight loss in density.
Ascorbate hydrolysis is in my experience negligible, I once had a 50g/L solution of sodium ascorbate in a full sealed glass bottle that was still active after 5 years.
Phenidone oxidation probably only occurs after all the ascorbate is oxidized.
Phenidone hydrolysis is pretty slow, I once had a full sealed bottle of Mytol that took over 3 years to lose activity.

It can be seen that in the 1 year sample the concentrate has gone orange, I suspect this comes from ascorbate oxidation and will run a test using sodium ascorbate solution with nothing else to see if this can be reproduced.