Over very many years now we have been doing multiple labelling immunofluorescence and we have tried several different compunds to try to address this problem. Every time we keep on coming back to something very simple - carbonate buffered glycerol. The sections do not have to be dehydrated so that no further distortion is introduced. We have solved the coverslip probelm easily and cheaply by sealing around the edges with nail polish. A complete seal also extends the storage life of fluorescent preparations, probably by keeping the oxygen out, and delaying the gradual increase in background fluorescence that often accompanies more prolonged storage.
Samples seem to last for weeks to years if kept in the dark.
I use a drop of superglue or some nail polish to prevent the coverslips from moving.