AFAIK, Mesmerize (lvl 4) and Mesmerization (lvl 16 AE) are identical, with the exception of AE flags being set and mana requirements. AE is not overridden by single target, which is a tradeoff (short duration AE versus long duration single) If you want to lock down a room of mobs, you can use AE, but will have to reapply the AE before it breaks, or root the mobs and use single target after the AE wears off.
Read
http://wiki.project1999.org/index.ph...nchanter_Guide for your answers about mez.
Edit: @nilbog what kind of research are you looking for? AE has never been overwritten by single target. As far as Mesmerize overwriting Dazzle, it may be a "feature" of Mesmerization allowing it to AE mez in emergencies. If Mesmerize IS using the same debuff as Mesmerization, then it makes sense.