"If I search for my username I now given 788 questions"
Before you would have got 50 and 50 only.
"dating from 24 Dec 2010 to 4 Sep 2012 (why that date range?)"
Because you signed up in 2010? There's not much chance of anyone talking about you before then? There are questions from 2013 in there.
"the earliest on the first page, whereas before this latest change it used to give me my latest question at the top or that 1st page with the rest in reverse chronological order."
In the SEARCH? Are you absolutely sure? Because we haven't actually changed the function behind the search - we've just displayed new information about the the quality of the "match".
"Answers are now similarly messed up, in an almost random order, and none after 2011."
Again, are you sure you're not thinking of your profile? Here:
http://www.theanswerbank.co.uk/user/stats.php ?