UPDATE 22 Jan 1553 GMT:
I received this from the original questioner:
Thanks for the tip, Father.
Between you and me, we ordered the mustum, put it in the sacristy fridge, and Father used it without saying a single word.
Back to validity again, Deo gratias.
From a reader:
What is the status of a Mass at which grape juice is used in the chalice?
Not mustum, not wine resembling anything close to the definition thereof, only pure, 100% Walgreen’s brand grape juice mixed with a drop or two of water.
Does this fall under the category of one element being consecrated without the other (i.e., the bread), or is the entire Mass invalid?
No other wine is consecrated, e.g., for communion of the people under both species.
We have a real, ongoing problem here with this.
So far, only the sacristan and I are aware…
Grape juice which has not undergone any fermentation at all, which is not thus at least mustum, is not valid matter for the sacrifice of the Mass.
For there to be a valid consecration you must use juice of the grape which, without additives, has undergone some fermentation (even if halted at a very early stage by, for example, freezing, thus called "mustum" – even with less than 1.0% alcohol in some cases, or – normally – having undergone more fermentation to become "vinum").
If you have expressed your concerns to the priest and get nowhere with him, then you must contact the local bishop IMMEDIATELY and explain what is taking place.
If that doesn’t produce results or any satisfactory explanation you need to contact Rome. Write to the Congregation for the Doctrine of the Faith, which has competence in matters of concrete instances of validity of sacraments. The same CDF also was the dicastery which issued the directives about the use of low-gluten hosts and mustum.
Remember: Some sort of proof needs to be supplied, so keep copies of what you very respectfully wrote to the priest and/or bishop and what they responded.