Alright, I'll let someone else answer the part about how the new Universal Express works. (Although it has been working GREAT!!! You're doing the right thing though, as one of the biggest issues we are working on is educating our guests on how to use the system)
On the crowds:
Typically, Easter Sunday and the two days following aren't nearly as busy as the few weeks prior to Easter (especially the week between Palm Sunday and Easter Sunday). Easter Sunday itself will be the least crowded - particularly in the morning (really only the Magic Kingdom gets slammed-packed on Easter).
Don't get me wrong, you will definitely have some 30 minute + waits during your visit - but if you are coming to Florida at all for Spring Break - the week AFTER Easter Sunday is one of the better choices