ad1. meta_values are strings, so they are ordered like strings, not dates.
In SQL you should use some casting as a DATE before ordering
ad2. better date format would be yyyy-mm-dd, because sorting this meta field like strings gives the same results as date-type column..
compare order of dates in yyyy-mm-dd
2010-03-06
2012-06-05
and the same dates in dd-mm-yyyy
05-06-2012
06-03-2010
both are sorted (ASC), but the first example is in correct order.