Ah, a shared darkroom!
Some chemistry is really robust - partially filled working strength solutions last well.
Other chemistry doesn't last well at all.
In some cases, undiluted concentrates are also quite robust, while others aren't so.
Each of the chemicals you refer to have either a Kodak data sheet or are referred to on a Kodak chemical table. For example, Tmax developer's data sheet is J86. Here is a link to that datasheet on Kodak's website: http://www.kodak.com/global/en/profe...bs/j86/j86.pdf
If you look at page 9 of that datasheet, it will tell you about storage conditions.
Here is another Kodak link that you can use to find all sorts of other useful links: http://www.kodak.com/global/en/profe...ankIndex.jhtml
For liquid developers, I would be reluctant to mix old concentrate with new, unless I had had full control of the old concentrate, and knew for certain that it was still fresh and fully active.
In a shared darkroom scenario, it is really important to take some steps to ensure that all the chemistry is in proper condition. There are a number of ways to do this, including:
1) each user uses only their own chemistry (probably not practical);
2) all chemistry is controlled by one person, who mixes up everything and carefully controls when it is used and when it is discarded; or
3) each chemical is clearly marked with the date that it is opened and the date which it must be used by.
Each of these methods requires that the information about storage life be known and complied with.
Here is another link that shows a variety of information about keeping properties: http://www.kodak.com/global/en/profe...3cp/e103cp.pdf.
From the last link, you will see that your already mixed hypo-clear was probably near the end of its life.
Oh and given how cheap stop bath is, I would never re-use it.