Mixing your dichromate in advance as you suggest should be not problem. Just be sur it is well labeled.
I have some more than a year old which I utilized yesterday.
Jim
No problem whatsoever with deionized water. I have a small bottle I once received as a contrast agent for Ziatypes (I don't now remember exactly when, but more than 6 years before) which is, for all I can tell, as fresh as ever.
Mix the dichromate with *distilled* water and it will last for years. I have on hand some 12-15 bottles of different strength solutions of both ammonium and potassium dichromate mixed with distilled water, most of which were mixed five years or more ago.
Don't contaminate dichromate solutions with tap water. Even small amounts may cause the solution to degrade within a few months.