We finally made it work for MW 1.26. These are the changes you need to make:
1- Mentioned earlier in this thread:
Near line 440 in SpecialUADMBase.class.php change the if to look like
$retvURL .= '&' . http_build_query($nonDefaultParams);
$retvURL .= '?' . http_build_query($nonDefaultParams);
2- Function isValidEmailAddr has been deprecated versions ago, so you need to change that call in SpecialEditUser.class.php and SpecialAddUser.class.php.
User::isValidEmailAddr( ... ) changes to:
Sanitizer::validateEmail ( ... )
and that's it.