MCrosby

David and Cliff, I appreciate your frustration.  Blood suppliers are well aware of the critical nature of their products and cringe each time they are compelled to supply less than the request number of units.
Does your facility have an active part in the acquisition and stewardship of these DONATED gifts?  I would think working with your supplier to find a way to increase the available inventory would be of benefit to all.  For example does your facility help publicize blood drives and or encourage staff to donate. Does your facility hold successful blood drives?
Do you monitor the utilization of especially precious products.  I cringe when I hear an O neg about to expire went to a non-Rh neg donor in order that it not be "wasted".  If an Rh positive unit would have worked, the unit WAS wasted because I bet somewhere in the system is an Rh negative recipient waiting for that unit. The literature suggests that 50% of the time this is how Rh negative units are allocated.
Please try to import Rh negative units on the open market.  At a meeting today one of our customers commented that to get Rh negative units they would need to purchase proportional equivalent number of non Rh negative units.  So for example if you want to import 7 0 negs you would need to accept 93 Rh positive products.  And this is the experience of the blood centers.  I think your accountants would fuss if your attrition doubled from the extra blood you had to take.
The low titer group O whole blood will take a toll on the availability of O positive blood.  Because the expiration date is 21-35 days depending on the anticoagulant we will be approaching O positive donors more often to meet trauma resuscitation needs.
This is the only business I can think of where the Product is voluntarily given by someone who will never know the good deed they did.  Unlike product manufacturers, we cannot go to a plastics company or a drug company and ask them to just increase their production to adjust for changes in utilization. We do not PAY blood donors.  We try to convince them blood donation is an honorable thing to do...and oh by the way we will give you a t-shirt.  As a result we are not supplying a commodity, although blood centers are treated like vendors and are compelled to bid against each other producing ever slimmer margins. Think about whether you would want to issue blood that said PAID DONOR on the bag. Historically this has not worked too well, but it might be a way to increase donor participation.
I suggest, rather than blaming the blood supplier, opening a dialog with them and being prepared to do some work on your part to help improve the blood supply would be more productive.  No one wants patients to die for lack of an appropriate blood product when transfusion is indicated.

Maybe I'm taking the simplistic approach but when we see mixed cell reactivity, we consider the 3 reasons given by the manufacturer for causing such results ...
1. Mixed population - this is not the case for screening/panel cells so that out.
2. Cold Agglutinin - you investigated that and got negative results so no cold agglutinin there.
3. Rouleaux - I'll assume you checked that out as well and found none otherwise you would have said differently. n.b. Logic tells us that rouleaux should be seen with all cells, but experience tells us that it doesn't always happen that way.
 
In addition, the premise for reactivity in gel is essentially that the cells that are 'not smooth' don't make it to the bottom.  Anything that causes the surface to be 'rough' causes a 'positive result'.  Irregular shape (e.g. sickle, acanthrocytes), abnormal immunoglobulin coating, etc. etc. so those things need to be added to that list.  (Keeping in mind that reagent cells are aged.)
 
Also, gel is acidic, so we see more Anti-M than we did with tube testing.  I'll assume you checked that out.
 
Anti-Pr ... aren't all human cells Pr positive?  However, if I remember correctly, isn't Anti-Pr also enhanced by acidity?  Again, your patient doesn't have a cold agglutinin.
 
Gel is also great at picking up HLA/HTLA antibodies.
 
Currently, with no cold agglutinin demonstrated, no rouleaux and a negative DAT, and 'some pos with some neg results', we'd run the antibody screen with OES (Ortho's version of LISS/Albumin) and if that is negative, call it 'HLA/HTLA Antibodies' and move on with our lives.
 
P.S. We do have two active patients right now who's plasma reacts with everything in gel ... negative DAT, no cold agglutinin, no rouleaux and totally negative with tube testing ... calling it 'interferring substance with the method' as we'll never know what is causing this .. meds? abnormal proteins?