I'm not sure about the bases if it has been years, I asked about this myself recently, and the general idea I got that a couple of years (the time frame I was looking for using them up) was probably OK. I seem to remember reading about this on BB/Soap Queen, maybe, and that the advice was to add a bit of glycerin in (but not too much, or it would get slimy) if the base seemed dry/splintered when cut, but not sure if there is some point at which it becomes unworkable ...
If I was making it for a basket for an auction I might just try and order some more really fast from someplace that is near you and ships quickly. Nature's Garden ships immediately (usually same day) but they are in OH, not sure if they would reach you in time. Anyone know if there are good MP base suppliers on in the NE US?
There are times when I have needed something really fast from a given company, placed an order, and then called to ask the co. to ship it out right away if they can, they are generally pretty good about that if you are nice and polite. I wish you lived near me, I could just give you as much as you needed to get this done.
You can use almost all micas w/MP. I don't remember any that were really bad in MP (there was one did not work v. well w/either CP or MP), they usually only morph in CP.