Now I strangely solved the problem setting-up UTF-8 only for markitup prevew, like this:
header('Content-type: text/html; charset=UTF-8');
instead of the default
header('Content-type: text/html; charset='.$cfg['charset']);
So, now I have my website headers declaration set to ISO-8859-1, my MySql database with collation and charser set to unicode-utf8-ci, my database datas (I think) inserted with ISO-8859-1 format...
but all seems to go well
The refresh problem has caused by an error (I think) in markitup.setup.php
autorefresh=01:radio::0 preview auto-refresh
autorefresh=01:select:true,false:false:Enable preview auto-refresh
Now, it seems to be in order...