This is my idea to distribute the blocks of the ones that are not making one for everyone. No matter how you do it, it will be a BIG, BIG job!
As the blocks come in, put the 'less then' full amount ones to the side. Record on their return envelope the number they sent. Put all the envelopes down with the return envelope under the envelope with the blocks in. Leave everyone's blocks in their envelope until you get ready to pass them out. If you do that you'll know exactly where you are if you have to stop.
I think I'd start distribution with the 'less than' full amount stacks. Take the first bunch of blocks, give one to the owner, then put one block down for each swapper as far as they go..(I think I may start with the full amount swappers when I did this part, that way you'll know when you've passed out all of the 'less than' blocks) Take the next envelope and put one on the owners stack and start putting those down where the last blocks stopped.. do the same with the next and the next.. as far as they go.
Once you've made the 'less than' blocks go as far as they will go, you start with the 'full amount' ones.. give each stack one, while watching how many the 'less than' stack gets. When they have the amound they sent in, put all the blocks in their return envelope and turn it face down.
If there are any blocks left over from a 'full amount' swapper put them into the return envelope of the owner. Continue picking up a 'full amount' envelope, giving everyone a block and putting the extras back in the owners return envelope.
I'd probably set the envelope of the ones I am currently handing out, out of line from the rest so I know where I am, and where the extras go.. Once you've gotten to the end of the line (HOURS later or perhaps days later) Everyone's blocks go into their return envelopes and seal them up! If they had extras, they should already be in their return envelope!
I think that would work.. all the full amounts would get one from each 'full amount' swapper and the 'less than' full amount swappers would get however many they sent in. Obviously not everyone will have a block from some people. Depending on how many 'less then' full amount swappers we have, we may or may not get one of their blocks.. I doubt anyone would get two.. just depends on how many of them there are.
Hope that's helpful.. if not, just disregard it.. If someone sends a better idea.. good deal.. use it! We have so many creative people that the best way to do this will come out sooner or later!!
I do think it would be a very good idea if everyone would put on the outside of their envelope the number of the blocks they sent.. That way you would be able to identify those with full amounts and those with less. Might make sorting easier.